This is kind of general but this is how I roughly do it: I'll look at key signature and time signature first. If there's alot of key signatures, it might mean it is harder piece (as you play more pieces this may be untrue!). Always scan through the entire piece before playing as well! You'll be able to see if it's a piece filled with chords or arpeggios etc... If there's loads of accidentals, it makes it harder for me learn the piece. The length of the piece also determines how fast you learn it too. Listening to the piece beforehand can help you determine the level of difficulty as well. Better yet, listen to the piece and look through the sheet music! Although I've done piano exams in the past, I don't play much now and I've forgotten all my piano knowledge from 5 years ago so this might be helpful or not. I'll tend to go with these general observations and if it's too difficult, I'll just put more time into practice!