The surefire way to tell is when someone shares their wisdom with you and you learn from it. Therefore I think it's actually subjective.

The greatest situation I've ever found myself in was mentoring someone that I was learning from at the same time. Bi-directionally shared wisdom is unstoppable, and in this particular case, together we dominated the 200yo, 5000 employee company we worked for while we were there, and drove it to perform about 5x.