If your standing over the bike with your feet on the ground, push down on the handlebars with your palms on the grips. Then adjust the rebound so that the bars say touching your palms when you push down and pull them back up. If you can pull your hands away the rebound is too slow, remember that you don't want a pogo stick fork but you don't want it too slow or it will starting packing up and make for a horrible ride.
For rear rebound I've found that if you know of a trail section will small little dips. Make several runs and adjust your rear rebound so that if you stay seated thru the dips it doesn't feel like your about to get thrown over the bars. Hitting the dips and having it slightly lift you off the seat is OK, you just don't want any sudden and fast returns that throw you up and out of the seat out of control. Normally you would probably be standing when hitting these dips but slowing down and staying seated will help you dial in your rebound to perfection.