Repeated, observable exchanges of items, especially where money is visible. Offers to sell you drugs or conversations about drugs that you overhear. Groups of individuals frequently congregating in the same area. Buildings where extreme security measures seem to be taken. Buildings where no owner or primary renter is apparent and no home activities, yard work, etc., seem to go on.
Many communities have unpopular residents.
However, obnoxious or peculiar behavior, different lifestyle, racial or ethnic backgrounds, or economic level doesn't mean that a person is a drug dealer. The rights off all citizens must be protected even while combating the war against drugs. Citizen reports can't usually be the primary cause for a drug arrest. Unless you have special training or experience with drugs or drug users, the courts will say an arrest based only on your testimony isn't justified. Since few citizens can meet the strict legal standards, officers who have the training and experience must make their own observations and collect evidence the courts will accept.