The easiest thing to do is to look at the pulse sequence directly. You didn't mention whether you're using Bruker or Varian (Agilent) spectrometers, but both TopSpin and VNMR/VNMRJ can display the pulse sequence graphically, from which it's fairly easy to identify the water suppression method. Alternatively, you can look at the text file of the pulse program itself. Many standard pulse programs have comments at the very beginning that give the general outline of how the sequence works, and the method of water suppression is often given as part of this header data. Furthermore, many pulse sequences are labeled by name (comments in the actual code) at the start of common blocks such as INEPT, etc. Maybe you can find info on the water suppression method there.

Unless you're running an exotic flavor of HSQC experiment, chances are you're dealing with a common water suppression method such as WATERGATE or a water flip-back pulse.

If it's a Bruker sequence, then there should be a file called pulseprogram in the directory containing the data. You can use any kind of text editing software (Wordpad, nedit, gedit, etc) to view the file. It should specifically say what kind of solvent suppression was used.