Since it's such a frequently asked question I've documented it here with some images which should hopefully clear up any confusion. The following screenshots show how to identify the Bluetooth stack on your Pocket Pc without writing any code or delving into the registry or system files.
Microsoft
Widcomm/Broadcom
Pocket PC 2003 Second Edition
Finally a quick reminder, the Bluetooth.NET library only works currently with the Microsoft stack, so if you have Widcomm/Broadcom or any other you'll have to look at third-party APIs.
Disclaimer The opinions expressed herein are my own personal opinions and do not represent my employer's view in anyway.