Stand at the top of a jump and watch other people hit it and see how they do with the amount of speed that they take into the jump. Also, for sure you can talk to people at the top of a jump line and just ask them how the speed is, most people will be cool and tell you how it is if they know. But even following someone else's tracks in the snow works, if they had good speed.
This whole business of rolling up to the top of the lip sounds great, but for me I know that I can't really get anything from that. I already kinda know "about" how much speed I need, I'm just fine tuning it, and watching other people is a tried and true method.
But whatever you do, don't let peer-pressure make you drop in before you know exactly how much speed you are taking into a jump. That's when you get hurt.
Formerly known as Jibbsta.
SLIDE METAL FTW