I use small shallow round dishes. I get mine from a lab glass supplier. Put water in them and shine a flash light on the water. Moving the dish around will show you the major resonance points. Make sure you don't overfill it so the water doesn't bounce out. You can also rent industrial strain gauges specifically designed for vibration measurements but unless you already have a degree in mechanical engineering this is overkill.
Measuring Vibration in Equipment
There are alot of threads on shelf material and tweeks that cut down on vibration. Is there a device that measures this vibration? I think I remember of hearing of such a tool before my audiophile days for testing crates for shipment but I cant remember what it was called. In The Complete guide to Home Audio they suggested using a stethescope. Has anyone tried this?