You can start simply by hooking up the piezo element directly to an oscilloscope that has decent sensitivity. If you do not have a scope that has decent sensitivity then start by creating a simple operational amplifier circuit to boost the output from the piezo to something a scope you have can read. Without an oscilloscope you could possibly use a PC sound card for input as long as you're within the human audio range, although you would definitely require and operational amplifier circuit to feed a sound card a high enough signal to read.