If you have caught him in an online liaison once before, you should assume he will try to hide what he does from now on. One way to do this is to install a spy app on his phone or tablet.
This will show you everything that he does on that device.
Ask her outright if you think she can be honest with you.
With a spy app, you will be able to track her locations which will help you to finally have your answer.
Unfortunately, if you suspect that there is an issue in your marriage the answer is probably already there.
He could be using different names on dating websites to protect his identity, but if you run the social profile software, you may get your answer.
I have picked up a few messages on her Facebook referring to "I have read your profile and am looking for my soul mate" also " a message referring to actually having found her good & desirable". There are other strange behaviors which could be relevant, yet which she frankly denies any cheating.