Kilesas can be found in thoughts, speech and deeds.
The mind is so volatile that it is difficult to spot them in thinking. By the time they appear in deeds, something has already been done and a karmic chain is underway.
That leaves speech. One can listen to others speaking and, more difficult, one can listen to oneself speaking and try and spot them there.
New Project: KILESA SPOTTING IN CONVERSATION
The aim in all this is to become an expert kilesa spotter so that one can get on with the job of ridding oneself of them forever. Once the impurities have been removed from the water, it is already pure.