Although there are several ways I could tell by listening to the amp, I can't describe it well enough for you to check it out that way. The easiest way to tell is to pull the chassis out of the cabinet. The circuit board is only a couple of inches on a side and sits on a stand-off post above a tube socket. It has a relay and a few other components on it, and some flying leads that run to several places on the main circuit board. The relay is about as big around as a pencil and an inch long. If you don't see anything like this, you don't have the relay mod.
Will