we use some equipment to test devices in the lab which is fairly expensive so would not be in routine use. There are some small and low cost ways to do a basic test using a 'water balance' indicator (I will paste example link below) but these will only give you and indication of approximate output - you will know if your device is working and in the right general power level but I would not count as accurate. Simplest way to test output - at zero cost - is to immerse ultrasound transducer in bowl of water, turn applicator face up towards the surface, put machine on continuous output, turn up to say 1.0 W cm^2 and if it is giving an output you will see water surface ripple/disturbed. Gives no indication of accuracy, but at least you will know whether something is happening and therefore whether some output.
Examples of high cost water balance test kit: http://www.acoustics.co.uk/product/radiation-force-balance/
Low cost units such as:
Not sure which country you are based in - might make a difference