Well, there's quite a few different articles at different sites about this, golfdigest.com for one.

Personally, I go into 'pro mode' when on the green (although it often takes 3 shots to get there).

I stand behind my ball, making a line from me to ball to hole (X-o-O), and get as low as I can before looking past my ball at the hole. If you've ever looked at 'magic pictures' (you have to look 'past' the picture to see it), you'll know what I'm talking about. This gives me an overall impression of the left/right breaks, and shows any switchbacks.

Then, I take a profile of my putt where I stand between my ball and the hole, often 20 feet away or more (o-X-O). This lets me know where the tiers are (if any), and basically lets me know if I'm putting uphill or down.

Finally, I crouch right behind my ball (Xo-O), and plumb-bob with my putter, looking with only my dominant eye, and bisect the hole. If half the hole seems higher than the other half, this indicates the 'final break' (away from the high side). I say 'final break', because this break will affect your ball the most as it slows down near the hole.