Every time Cassie or one of the other Merriwicks greets someone without turning around.
The fact that every single person in Middleton is unsurprised by this, implying that the whole town know that Cassie is a witch and only Sam and Nick are ever surprised by it.
Specific Episodes:
Episode 2x10 has Stephanie storm into Bell, Book, and Candle and announce to Cassie that she's not talking to her. Bemused, Cassie points out that Stephanie is talking to her right now.
It's because she was helping Cassie plan a surprise party for Martha. Also, Stephanie's complete inability to keep it a secret. This leads to her telling Abigail, the latter letting it slip to Martha, then telling her that it's actually Cassie holding a romantic dinner for John. Which leads to John hearing about it, and being befuddled since she knows he's going to be out of town at the time.
Then, Stephanie storms into Bell, Book, and Candle with the above line, and Cassie has to explain to her that there is no dinner. Stephanie then admits that she let slip about the dinner to John, while gloating that it's all Abigail's fault, not hers (although, as Cassie points out, Stephanie did tell him about the dinner, even though she didn't know it was fake at the time).
Stephanie, trying to remain clueless about the party:
Stephanie says that she has a couple of cakes she was going to bribe...er, offer...to town citizens (there was a contest for best-looking business). Doubles as a heartwarming moment, since she invites Abigail to join her.
Grace walking in on Sam and Cassie snogging, then spinning around and walking straight out with a mildly traumatized "nope, not dealing with this right now" expression.
At the beginning of series 3, Abigail is dating three guys at once. Cassie worries that this might be a bit much, but Abigail brushes it off. Sure enough, though, all three of them meet, and after some discussion, they go out for drinks while leaving a befuddled Abigail behind.
This conversation from 3x4, after one of Sam's patients comments that he's the smartest doctor in town (this after Sam prescribed a recipe of Cassie's, at Eve's advice):
Later on, George says, "I used to think my poker face was pretty good." Everyone else tells him—in unison—that it's not.
Stephanie is venting to Sam that "gourmeteater428" left a bad review online about her business. Sam suggests checking her records to see who signed their bill "Gourmet Eater." At Stephanie's face, he nods that it's probably not their legal name.
Martha's disgusted face upon walking into Bell, Book, and Candle in "In Sickness and in Health" and seeing Cassie and Sam kissing.
Martha: "Oh, for heaven's sake. All right, you two, we get it. You're in love! A little restraint, please?"
While Martha's talking to Cassie, Sam comes over with a bag of ginger candies (for a patient of his). Martha throws it significant glances until Sam asks if she wants one.
Martha: Oh, I couldn't possibly. But since you offered...
She then proceeds to take the entire bag and head off.
3x11 Not Getting Married Today: Part 1 has Cassie visiting Abigail in her shop. Two enormous peacocks on plinths, one black and one white, have been delivered to Abigail. What follows is a conversation where both of the Merriwick woman have absolutely no idea what on Earth to do with the monstrosity before them. Fortunately, right on cue, Martha turns up and promptly orders a whole dozen of them before remembering she's allergic to feathers.
The first episode of series 4 has Nick coming up with a variety of ways to scare his dad. None of them work...until he gets locked inside a cabinet for a few hours.
At Cassie's bachelor party (It Makes Sense in Context) she absolutely creams all the other guys at pool. Meanwhile, Sam has a hen party where Eve, Grace, Abigail, and Stephanie wear aprons with his face printed on them.
At Sam and Cassie's wedding, the minister says the traditional "you may kiss the bride" line...only to realize they've already started.
Sam is clearly having a lot of fun Twerp Sweating Luke. The whole scene is both comedy gold and a classic example of the trope, complete with crushing handshake, Death Glare, and this fabulous bit of dialogue:
Luke: Heading to the roller rink...
Sam: And?
Luke: And we're gonna skate...maybe get some food...healthy food?
Sam: Good choice. I remember picking up Lucy Carmichael on her first date. Her dad was a lumberjack. All I could think about was him chopping down trees.
Luke: You're a doctor, right?
Sam: Surgeon. I cut people open.
He then cheerfully admits that he's just having fun, to Luke's visible relief.
Just before the wedding, Grace eagerly asks Brandon to tell her everything he knows about tormenting a younger sibling.
Cassie suggests to Sam while they're at the lake house that they should enjoy the view.
In 5x07, Nick comes upon George looking through a photo album at his "high school sweetheart." Nick agrees that she's a beauty. And then we see they're talking about a car.
While Adam is looking at the "Little Book of World Records," a Freeze-Frame Bonus shows us that the record for most straws used at once is 183.
Sam asks about Abigail's Halloween confections...for the kids.
Martha announces that for the Halloween triathlon, they'll need someone courageous...(George looks at Nick)...dedicated...(Nick looks at Abigail)...selfless...(Abigail shares a look with Cassie)...and strong (Abigail and Martha share a look as Sam walks in).
Sam: Sorry I'm late.
Martha: Not at all. You're right on time.
George: Congratulations.
Cue an utterly befuddled look from Sam, as Cassie smirks.
Martha does a roulette to determine what the Halloween triathlon events will be. Cassie announces the first before Martha can even look. Abigail announces the second, and both women smirk at each other. Then Martha holds up the third...and Sam announces it before she can. Everyone swings to look at him in shock.
Sam: I saw it reflected in the mirror.
Sam is waiting for Cassie to come down the stairs when the doorbell rings. He goes to answer it...and she's on the other side of the door. Despite the fact that the only known exit is the stairway Sam's been standing in front of.
When Nick comes home in 6x03, he explains that he has a big project to work on. Sam notes that dorms can be distracting.
Cassie: So can girlfriends.
Note that there's no way, judging by Nick's surprise, that anyone could have told her.
The following episode has Tom pounding in a sign that says "Tinsdale for Mayor." Then he steps aside, revealing that it says Tom Tinsdale. And later it turns out he was just doing it to mess with his wife.
Martha's name for her chilli: "Chilli Chilli Bang Bang."
In episode 7x06, George is fretting over his upcoming date with Samantha.
Cassie: Having second thoughts?
George: And third thoughts, and fourth thoughts.
Cassie: Oh. One of those thoughts wouldn't be throwing the towel in, would it?
George: Might not be a bad idea.
Cassie: Maybe you're right.
George: Are you saying I shouldn't ask the question?
Cassie: It sounds like you're saying I shouldn't ask the question. I'm just agreeing with you.
George: No encouraging pep talk?
Cassie: Eh, what's the point?
George: (standing) The point is, I might change my mind!
Cassie: Have you?
George: Darn right I have!
Cassie: (innocently) Well then, I'm glad you did.
George: (with a slowly dawning expression of realization on his face) Looks like I just got Cassie Nightingale'd.
Cassie: You're welcome.
As Abigail and Donovan are about to snog, she blurts out, "I wish I could kiss Stephanie." She clarifies that she just got a flash of what Adam (standing right next to them) is thinking.
If Joy is telling the truth, she once tried to fly a broomstick, but it got caught in a tree.
Martha talking to Tom on the phone, only for him to walk into the office.
She ordered a chandelier for her housewarming party, but it's a tiny model instead of a full-sized one.
Martha: Well, it looked full-size online!
Sam's mantra when Cassie makes him meditate: "I'm bored...I'm bored..."
