As people told you, I don't think she is, or rather, I don't think you should base your suspicion on her dressing nice for the one day in the week they are having an office meeting. That makes perfect sense to me, when I worked at an office i always dressed nicely for a meeting. It gave me more confident to speak out loud and be more assertive.
I do think, however, that you have a trust problem, whether it's justified or not is another matter, but either there are more things that makes you suspicious or there's a problem in the relationship, or you have some problem with yourself. I'm not even sure asking your wife if she's cheating is a good idea, it's very insulting to be suspected for no good reason, but I do think the two of you need to address your lack of trust.
The best way to do that, would probably be at marriage consulter office. It seem to me that two of you aren't very open with one another and that's a good place to learn better communication skills.