is it legal? can they find out? I would find this useful too and WAH doesn't live with me and isn't being 'straight' about his A.
Depends on your state. In most states, it's perfectly legal to monitor a computer you own (or co-own), and/or to monitor your children's activity on it.
Capturing a spouse's passwords, when they "have a reasonable expectation of privacy," and then using those passwords to access their banking info, for example, is another matter, although there are legitimate defenses against that as well.
