Measuring that horizontal speed is tricky. Motion detectors don't pick up objects as small as marbles very well. I suppose video analysis would work, but that's too intricate for a general physics class, I think. In the past, I've had the class use stopwatches. If the marble rolls across a flat tabletop, then the distance of the flat region divided by the time to travel that region gives the marble's horizontal speed. However, my lab groups have made consistently incorrect predictions using this method. Just a small reaction time issue can cause the marble to miss the target by 30% or worse.
David says he uses a photogate placed near the end of the table! Knowing the marble's diameter and the time during which the gate is interrupted, the marble's speed can be calculated. Even better, use two photogates near each other: the speed is the distance between beams divided by the time between the beams' activation times. Reaction time or stopwatch clumsiness is not an issue when photogates are involved.