What specific pieces of evidence do police need to charge someone with a drug dealing charge?

You do not require "direct" evidence to prove a crime.  A conviction resting solely on circumstantial evidence is sufficient.  Classic example:  Baby is heard crying. Man walks into the room, kills baby. Baby is no longer crying. Witnesses see man walk out with bloody hands.  No direct evidence is necessary.  A second example is if there's text messages from you saying "Come over, I've got 5 grams of Cocaine", and people see your client come over. Immediately after leaving, they find 5 grams of cocaine on him.  Its reasonable to say that you are the dealer.