At least in a general sense of what to look for. Like a general "X can influence the flow of mana.", "X can assist your bargaining with servants of chaos.", or "X can assist your stealth." Don't know if those powers actually exist, but examples. No need to specific exactly what it does, but I'd know what to look for.

Also, to avoid people just dashing up to 17 to check, I could see it being like prereq, in that you can't get anything pre-15. Low levels, you barely know your familiar. By the time you're higher level, it seems more reasonable that you'd be able to tell what it does.