Secret test for a prospective spy

In my story, a British soldier and veteran of WWII is being recruited to be a spy. He agrees to meet the agent who will be his mentor in a cafe.

The person he meets isn't his mentor, but someone sent by the mentor as part of a ploy to test his stuff, see if he has the making of a spy or not.

The soldier passes the test, and the mentor reveals himself.

I'm drawing a complete blank of what the test would be. Any ideas?

It should be something simple that can be staged in a public area in 1948.

I'm assuming your spy will be going up against Russians. If not, you can adjust this scenario accordingly.

The first contact works for the mentor but pretends to be a Russian agent. Not overtly—the candidate must deduce this for himself during the meeting. The contact may subtly mispronounce a word or get a fact wrong, raising the candidate's suspicions. He can then ask a leading question to confirm those suspicions without alerting the contact. That's stage 1.

Once he realizes he's meeting with a Russian spy, the candidate must deduce that the mentor's organization has been compromised. That's stage 2.

The next step for an agent, of course, would be to follow the contact once he leaves the meeting. That's stage 3.

Ostensibly the meeting is to size up the candidate in person and give him the address of a particular office. If he fails any of the three stages, he will go to that address, which of course is bogus. To get there, he must turn east upon leaving the meeting place. The contact will go west. If the candidate passes the first three stages, he will go west as well.

The fourth and final stage is in how well the candidate follows the contact. A few blocks away, the contact walks past the mentor and gives a signal. One signal means the candidate is bad at shadowing, upon which the contact shakes him. Another means he's good at shadowing, upon which the mentor reveals himself. At no point can the candidate learn anything about the mentor or his organization (the contact has false ID).

The obvious exploit is that if the spy is really good, he'll realize that the contact is only pretending to be a Russian spy. But I suppose that wouldn't necessarily change his response.

Well, if he realizes it's a trick, he'll play along with it to show he understands what they're doing. Another possibility is that he believes the Russians themselves are trying to recruit him by pretending to be English. But the OP has already said that the point is to have the British recruit him. I'm trying not to overthink it.