I will offer you one VERY simplistic clue, speaking as a guitarist. I expect to get roundly criticised for this offering, just because it is so simple. And I'll state it here first: It's not 100% accurate. But here goes:

Say you are playing in the key of "D", for example. And you find that the melody goes to a place where a "C" chord" fits best. Or another example. say your are playing in "G", and you find you need to use an "F" chord.

That would be a definate clue. I hope this helps, Bob