Generally the only time a woman KNOWS she no longer loves her husband is when she has already fallen in love with someone else.
You know she has a crush on the married guy at the office so how do you know it's not already a full blown affair?
You can't ask her....if she's cheating, she'll lie. ESPECIALLY when the affair is with a married man. She'll lie to protect him and their relationship from being broken up by his wife.
Maybe I'm wrong but the only way to know for sure (and it's important to know for sure because a walk away wife is completely different from a wayward wife) is to verify whether she's cheating independently from asking her. Don't even let on that you are suspicious....just buy a voice activated digital voice recorder (use cash and hide the receipt and don't put your voice on it in case it's ever found and return it for a refund within 30 days at most stores) and hide it in her car. Should take long to catch a conversation or two between her and the likely OM or her and her "in the know" girlfriend.
This is not snooping, rather an initial important verification step to independently verify and ascertain the truth about your life.
Never divulge the source of your information. If you discover an affair, you aren't obligated to report how or what you know. If the recorder ever got discovered...it wasn't yours.
The internet is 90% complaining and entitlement and I hate it because I deserve better!