Personally I have made available the rankings for peon, minion etc as well as for routine, easy, etc. and give enough details for the Fated to determine how important who or what they're intarcting with is.
This means only I know the exact number but the Fated can still get a decent idea of what the TN is.
If they're interacting with a recurring character, chances are they have a good idea of his stats and can cheat accordingly. If it's not a recurring character, they'll only cheat if the challenge is important, and important challanges justify using high cards to succeed, so not knowing the exact TN isn't problematic. If it's not a character but a situation, they have a rough idea based on how routine/difficult the task is for them. Again, if it's important it warrants cheating high, otherwise it doesn't.