I would just program a different speed dial on each of the buttons from the handset, then go into WinSPM, Centralized Telephone Programming, and click on each button until you figure out which buttons are which. (or do a print of the extension info, which will show you which buttons got programmed)
Or, if you can figure out how to program the buttons from the phone itself, put a different speed dial on each button from WinSPM, then check the phone to find out which speed dials you ended up with, to determine which buttons are which.