Well this was most likely answered in the theory forum somewhere, but I believe I can do a pretty good job of answering your question. Yes the previous replies did point you in the right direction. If the song starts on a G major the odds are good that the song is in G major. But the way the chords work with the other chords is an excellent beacon. In a major progression, the chord flavors (we'll leave it at as a matter of major vs. minor) will foloow a pattern. In a major progression the I chord (the chord that is the key of the song) will be major. The ii chord is minor. iii is minor. IV is major. V is major. vi is minor and vii is diminished. Heres an example. If your progression starts on a G major. You could play a B minor but not a B major if you want your progression in G. So if that helps im glad. But if that didnt work, I have one last piece of advice. If you decide it's in G major. Your lead should be in G major. The seven notes you would play in G major are G A B C D E F#. But the major pattern is not as easy for beginners to make leads as the pentatonic. So then what pentatonic could you play? Heres a great revelation. If a song is in a major key. Find the sixth note. In G the sixth note is E. Then play in the minor scale of that note. E minor is the same as G major. It is the same seven notes. And the pentatonic is 5 notes but they all fit. So play in E minor pentatonic in this case and have fun :)