with which tool do we measure the amount of electrical energy?

Starting with a thermal coupler, it is just a verable restance that is tempature controled. So there is no real emf generated by it's action. It just (do to it's changing restance) causes something else to respond. An example being a cars temp guage. The temp of the engine starts to heat up and the changing restance of the coupler causes the temp guage to respond. In the case of the cristal, if struck, it can produce a voltage that can be measured with a volt meter, as in the case of an electric cigerette lighter. The cristal also is an osilator at some frequency and when excited, can be measured with an osiliscope or volt meter. Neither one are much of a generator for real power uses except as already being used.

You wouldn't use a voltmeter or ammeter unless you could find one that measures micro/milli volts.

If there was an instrument available I believe it would cost thousands of USD. Most energy produced by thermocouples and crystals are amplified to a useable value, then measured and the amplifiers are also very expensive. I believe you have a real costly project ahead of you.