When profiling civilians using hacked cameras or your smartphone's Profiler application, you may come across a potential crime event. You can also trigger this if you happen to hack into a civilian's phone and intercept a conversation leading to a potential crime. Head to the marked location but just enough to hide yourself and use the Profiler, preferably through the eyes of a security camera to identify the potential victim or criminal. Make sure you don't get detected otherwise the potential criminal will back down and cancel his plan. Once you get the ctOS Alert warning, head to the area and wait for the ctOS suggested action. Sometimes, you have to intervene while sometimes, you have to tail the potential criminal first before intervening. Catching the criminal without killing him gives you a greater reputation reward. Completing crime detection activities also give you exp. You'll also get automatic ctOS potential crime alerts once you've unlocked the ctOS tower in the area.
You have to be careful of witnesses though, especially when you're pursuing a suspect with a gun. Once he starts shooting, civilians may report to 911. You can stop them from completing the call by threatening them by pointing your weapon at them or knocking them out. If their call completes, a ctOS Scan will begin and police will arrive shortly. If you have the Jam Coms skill, it is a perfect tool for this situation.
You only need to complete 20 Crime Detection events to get a Progression Reward. After that, Crime Detection events will be a good source of experience and boosting up your reputation.