How many people in your life can say, "I'm proud of you," and you take it fully and without any sort of resentment or dismissal? Whoever those people are, they are probably your mentors.
Someone who credibly says "I'm proud of you" usually has two characteristics. First, he is probably higher status / higher power. Most of the time, having pride about someone else comes from a place of superiority. Second, he must know you well. Most of the time, to be proud of someone means you know where they've been and how far they've come — pride is a word about growth. If a homeless guy on the street (lower status) or Bill Gates (don't know him personally) tell me they're proud of me it won't have a huge positive effect.
To be sure, "I'm really proud of you buddy" can sometimes occur between friends. But this seems less common. Usually friends say "I'm so happy for you" or "Really nice job!" but not the p-word. And family can often be proud, but as with most things family, the obligation and bias dull the effect.
This topic came to mind because I recently saw a friend / mentor and told him about a meaningful professional accomplishment. The next morning, I woke up to an email in my inbox that was one line: "I'm really proud of you." It felt great, and as he falls into both of the categories above, was fully appreciated.
It got me thinking, "How many people could send me that sort of email?" And that's how I arrived at the "I'm Proud of You" litmus test.