If you suspect this it would likely be a good idea to blow out the drives (format) and reinstall everything, update all passwords to everything.
A person that knew what they were doing with physical access (and in some cases remote/online access) with the right privileges could have installed something that you aren't going to find outside a forensic study of the system drives.
edit- and just being completely fair about the implications....according to who you are and what you do, if someone had physical access to your location there could be additional devices installed in or near your location. This assumes that the effort, expense, and legality of said were worth the cost and/or risk based against what information you (might) have that they want. If this is a "cheating spouse" type thing, and I am not assuming anything, then the blow out should be sufficient to ease your concerns.