The Obvious
Step1
Let's start with the obvious. Do you remember where you last put an item and is it in the same place that you remember leaving it?
Step2
Do your items seem to be in a disarray as though someone has shuffled through a stack of papers but forgotten to straighten the pile back up? Or straighten it to the same way that you remember leaving it?
Step3
Do you notice if items are missing? If so, check other locations and try to rule out any chances that you may have actually moved the items last.
Step4
If the area is dusty, identifying moved objects becomes easier to detect. You will be able to identify that an object has been moved if there are any spots where there are no traces of dust or less dust than other surfaces.
Step5
If you don't see visible proof but you strongly believe that someone is going through your personal items, ask them in a manner that will not put them on the complete defensive. Say something like, "I have noticed that some of my papers are not where I put them last, Have you by any chance moved them looking for something?" This is less offensive then saying, "Hey! Are you going through my stuff?" which may result in your culprit bold-faced lying to you about it.
The Not-So Obvious
Step1
The hair test works great on catching snoopers who look through drawers and cabinets. Simply take a piece of longer hair or transparent thread, tape one end to an incspicuous corner/edge of the drawer or door, and tape the other end to a base of the furniture or doorway. The hair/thread will be hidden from sight so the snooper will not notice it. Once the drawer or door is opened, the hair/thread is unnoticeably pulled away and you have instant proof that the object containing your possessions has been tampered with.
Step2
The more particular you are with the way that you arrange your personal affects, the harder it will be for the person going through your items to return them to the original state. For example, if you arrange your canned goods in a way where the labels always face the same way and the first letter of the name of the brand is lined up with the can sitting above it, someone rummaging will not pick up this detail but you will instantly notice.
Step3
If you suspect someone of rummaging through your belongings, casually bring it up in conversation. You could act like you misplaced an item that one would only know about if they were snooping through things and see if they can "help" you find it. If the person goes right to the object, then chances are, they have seen it there before and possibly without your permission.
Step4
Bring up the topic of rummaging through personal items and watch for the person's reaction. If you simply state that you think it is wrong to go through personal affects without the owner's permission and your normally laid back friend changes to a brilliant shade of red or white and becomes jumpy, then chances are, he or she is admitting guilt through body language.