Hi there and welcome
your not going to be able to find out by checking sites as they will always use a pseudonym and you have no access to any personal details via these sites.
But there are other ways to find out.
First, does she use your computer?
ok, its not going to be possible, first there are thousands of sites, and then you really do not know which name she is using, then you do not have access to the data to check. They do this on purpose in the event a spouse is cheating.
you can check if she was on the Ashley Martin site as that was hacked, here
But only because it was hacked can you check this, the rest not sorry
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