You know, that's an interesting question.
I don't know if there's anything you can explicitly do-without breaking any laws or seeming loco-to findout if you're man is on the DL. I think it's one of those things you are eventually told or you discover. I mean by definition, if he is, his entire persona is intended to keep it from you or from being something easily detectable on the surface.
I can't encourage you to snoop through his emails or history folder on his computer. For a few reasons: 1) I think its in poor taste. You married him and he married you. There has to be some level of trust; 2) It actually is a felony and we don't promote felonies here at Guyspeak; and 3) I'd hate if somebody snooped through my stuff and I'd honestly ditch a chick if I found out she did, no matter what her reasoning and whether she found what she was looking for or not.
Here's my question to you: why do you think he's on the DL? What has he done for you to think he might be? And why are you typing it out in all caps? I understand you mean business here but I need more information. Perhaps you might need to trust your husband unless he's given you a good reason. And if he has, then you confront him. If he says no, he's not and you later find out he is, you can divorce him as a liar, etc. But you'd better be ready for the thunder if you bring that up to him, because no man is going to take that well.
No man.
Put a legal keylogger on your computer