I have always believed that writing a reference letter is similar to writing a love letter. You praise and brag about how wonderful this person is, and overlook all their flaws. Your language changes from daily speech, to a more flowery prose.
The more substantial proof is in the writing process. You start off by picturing their face; regardless of how they look, you start attributing their positive traits onto their physical feature. The eternally optimistic person will have their smile etched, and the corners of their eyes wrinkled, the sports enthusiast will have a trim physique, minimizing the body fat content, the perpetual green thumb will have that adorable streak of dirt across their forehead, and a slightly messy hair. Then the visualization comes to life. The modest kid shows a quiet inflection in his speech. The emotionally expressive person would be offering a hug. The homely would smell of fresh baking.
A good and truthful reference always gets better at the end. The process of writing builds this person up, which just further motivates you to write a reference that is worth such an amazing person. The ultimate test really is to finish writing the letter and meet up with this person, and not be disappointed. Sometimes we just build someone up so high that even the most perfect of men would fail to meet our standards.
The man I wrote my love letter to met my expectation and then some. =)