Where can I find ultrasound machine regulations?
FDA has got a guidance document for ultrasound systems and probes.  There is also the IEC 60601-2-37 standard (it can be difficult to read and follow).

The testing is done using one or more of the following methods: hydrophone, schlieren photography, radiated force measurement.

There are companies who specialize in making measurements for the safety certification.  Large equipment manufacturers might have their own testing facilities.