Speed checking is just something that comes more naturally the more you hit big jumps. Usually I cruise into the jump, then hockey stop on the takeoff or on the side of the jump, and give it the best guess about where I would have landed. There's nothing really scientific about it.
Usually there's at least another person or two who have already hit the jump, and you can watch their run into it. No shame in asking what the speed is like too. But getting good at correctly giunea pigging jumps just comes with time and knowing "this is how the speed usually feels for a mellow 40 footer", "this is how it feels for a kicky 60 foot step up", etc.