Making a fake account to test a friend’s loyalty is the betrayal you were trying to catch.

Setting a trap requires you to do the thing you are afraid of first. You lie about who you are, you engineer a situation that would not otherwise exist, and you invite someone to fail a test they never agreed to take. Whatever they say to the invented person, you have already broken the thing you were checking on.

If the worry is real, say it directly: ask whether they said something, and listen to the answer. That conversation is more frightening and much more likely to end well. If you cannot bring yourself to ask, that is the part worth looking at.