Always look at the land around the green. Greens are built to drain water, so slope on greens typically will be toward a low point, a creek, pond, catch basin, etc. Feel the green with your feet as you walk around your ball and stand behind it–they can tell you slope on the green. If you can plumb bob or use Aim Point–that will help too.
Speed can determined a lot on the practice putting green before playing–all the greens on a course should be about the same stimp reading. Before playing don’t aim at any hole rather putt 25-30 foot putts back and forth, up and down across the green just trying to roll the ball up to the edge of the fringe. After about 12-15 putts you should be able to start figuring out the pace of the greens that day. Remember if you play early as the dew burns off the greens will tend to speed up some.
Lastly is grain. Bent grass really doesn’t have any grain that will affect putts. Bermuda does–so reading it is key. Several things to look for: Grain typically grows in the direction of water run-off and also the setting sun. If the grass looks shiny and silvery you are looking down grain (putts will be faster) if the grass looks darker and dull you are looking into the grain (putts will be slower). You can also look at the hole–if there’s a ragged edge on one side the grain is growing toward that side.
One thing to remember, faster putts are hit with less pace and the break takes more effect. Slower putts will tend to break much less.
I always try to over read putts that have a lot of speed and break a lot. When you do as the ball gets near the hole it’s always rolling toward the hole. If you under read it–it’s always rolling away from the hole. You might end up with an 18″-2′ downhill putt but to me that’s better than a 5-6 footer back uphill.