A small mass object moving fast can have as much kinetic energy as a larger object moving slowly. 1. Make a V-shaped catcher from cardboard, a 5x8 index card, etc. Tape the lined paper and the ruler down to a flat surface. Mark the starting point for the catcher, making sure it is exactly in line with the ball the marbles take. Flick the marble along the track and time how long it takes from when it leaves the track until it hits the catcher.
Precise timing is tricky.
It will take practice. The longer the distance from the ruler to the catcher the longer the time is and the easier to time accurately, but the more the marble will slow down due to friction. If no stopwatch is available, rough qualitative comparisons can still be made. Note how far the catcher is dragged by the marble. Note the effect of varying speeds and masses.