sign into their account, go to their timeline and click on the "activity log" button near the bottom right corner of the cover picture.
Check the box that says " There you will see all their "ONLY ME" acitivities, such as name searches, statuses, pictures, etc.
Then if you look on the left side page, click "Posts Youve Hidden" and it will show ALL the posts they have hidden from everyone on their timeline.
"Likes" shows ALL the likes that they do.
"Comments" obviously shows everything they commented on.
Get where I am going with this? The Activity Log button is the magic button when it comes to figuring out if your SO is cheating or doing anything they are supposed to be doing. So get to it ladies and come back and report your findings.