For checking the likes of hollow displacers for hot air engines, put it in a pot of hot water, if it has a leak it will blow bubbles. I'v never tried testing the whole motor, there might be a little leak at the displacer rod gland, and the power piston(even less). For air pressure, all you need if you go that way is a bit of tube, and blow, lung power should be ample.
If you find that you have a bit much air leak at the piston, try fitting a light leather cup seal on top of the piston (like the leather cup seal in a bike pump)
Ian S C