Robbing can best be deteched by watching the bees that come and go.
If you watch your normal hive you will that your bees return flight is almost too low to hit the entrance. You will also notice that they tend to enter almost immediately. When the foragers leace you will notice that they gain altitude almost immediately after leaving the hive (this is because they are empty and light weight).
If you watch robbing, you will see the bees come in high and linger (as if to debate if they think they can get in). You will also notice that as robber bees leave, they leave loaded down and almost drop to the ground before regain altitude (because they are loaded with honey).
Try to observe you hive at various hours of the day also.
In the morning most of your bees should be out-bound, if you find a large number on the porch in the early AM, it is likely you have a robbing situation.
The next question is how do you control or stop robbing.
You can do the sprinkler trick as mentioned above (this drives the bees home as if it was raining (but this tends to drive up the humidity in the yard too and this isn't good for honey evaporation. The best way is to try to prevent robbing in the first place. In the ideal work, your hives would be about equal strength. But since this isn't the case you can either manually equal the hives by pulling and trading frames or you can reduce the entrance to a small enough size that it can be reasonably guarded. I don't like trading frames around too much, its a good opportunity to spread disease, so I'd advise that you reduce the entrance (especially if the hive is known to be weak, a split, or a swarm that is slow to take off).
I heard once of someone taking all the covers off their hives, soemone told them that would prevent robbing? I don't subscribe to that logic, but would I might do is open the entrances of stronger hives to compromise them when they are robbing other hives. This forces a few more bees to stay home and guard the front door so less are out stealing.
Robber screens also seem to work from what I;ve heard from other people. I'm not a big fan of extra equipment so I haven't made/used any to really comment. IF you use good common sense about your entrance reducers and if you have other good cleaniness, you'll normally chase it off before it gets to be a problem.
Robbing is often caused by other forces too.
You have to maintain your bee yard pretty clean.
If you have loose comb about the yard with honey in it, it can cause robbing. And if you are sloppy with filling feed jars (espcially if you are using honey be healthy or other oils/smells to attract the bees). I've not been spilling as much in previous years, but if you have to fill a lot of jars, it might be advisable to take an empty 5 gallon bucket to fill jars over (that way the spills are caught). A little trick is to use the spills in your watering hole if you provide one openly in your bee yard (this way its diluted and it takes a lot more trips with much more water to steal equally what they would ahve gotten off the ground raw).