Internet dating site history
You can usually browse the selections on these sites without having to sign up for an account.
If it is your husband you will know for certain then.
Facebook users can see a listing of friends of other people even if they aren’t part of their friend list by choosing “see friends” when the screen appears.
This will at least let you see who he is friends with.
Using the browsing history you can now look to see if there are any adult sites or dating services, such as e Harmony listed.
If you find those, you have found your first clue that he might be participating in online dating.
If a dating site is listed in the history, the next step will definitely give you the answers you are looking for.