Hi,
The machines I've used have actually had a thumb wheel with RPM printed on it. Just turn the wheel to the speed you want.
Of note, I often find that I don't really look at the actual RPM, but make a judgement on speed based on the work. I know that's a wet fish answer you can't put your hands around, but you'll very quickly get a seat of the pants feel for it. As already mentioned, slow, not fast. Keep it moving, work up to higher speed, not the other way around. You can burn the finish in a second if you use too much speed/pressure/don't keep it moving - never let the bufffing wheel hit the surface at a severe angle or the edge will put a burn in lickity split. Now, don't be scared of the machine either. Any chance you have a hard dinghy or an old beater car you can practice technique on? If not, start someplace like the overhang in the rear quarter near centerline where you can reach it and if you have an "oops" it will be near impossible to see without swimming....
-David
Independence 31
Emerald