I thought that PBT sinks MORE QUICKLY than ABS.
When I clean them I dump them in a bowl of hot soapy water, and I never have a problem with either type floating, except for a few stragglers that I dunk with my fingers.
A sure test is to swirl a drop of acetone on the underside of the key and see if it leaves a mark. It will not affect PBT.
Or, boil them for 5 minutes and see whether they deform.