Welcome to Zelaron!
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To answer your question - first of all, we're not allowed to talk about hacking, but that's alright, because what you want to do is nothing near hacking.
Basically, you can just make an account on that forum, as you did here, and read what's going on. Obviously make the account completely unrelated to yourself to avoid confrontation (for example, go to somewhere like
www.yahoo.com and make a new e-mail account specifically for that forum, and register using false details).
Just a word, though - whilst I know nothing of the situation, now have I had any experience of it, make sure you have solid evidence before doing anything. A suspicion is just that - a suspicion. Unless it is grounded in indisputible evidence and fact, then leave the rash decisions out of it. You don't want to see a marriage of 14 years ruined unnecessarily because you had a
feeling.
If you want to discuss it more, then feel free to post it in the
Life Discussion forum. I can assure you that the replies you get will be serious (and if not, then I have the power to delete them).