When I was still struggling to play with just one hand, while the other hand impatiently wish it knew which key to hit, I wish I had stumbled across a Youtube like this. Germaine Griggs sent me this link in the e-mail this morning. I'll let him know that I am sharing this lesson with all my cyber friends!
Of course, there are several ways to find the keys. What I currently star with, is the first verse of the songs.
Another way is, when you play the melody, observe what keys are hit the most. That tells you what are the chord to play.