You can't handle a call until you determine what it is. Once you figure it out, it's a question of following procedure, protocols, and filling out the appropriate paperwork. The logistical side of the question is easy. The figuring out what you have side? Not so much. Rest assured, it gets easier with time and experience.
Remember that there may be some instances where the only action you have to take is writing an information report to document the incident. When to write a report is a very common question. A good rule of thumb is if you have to ask if you need to write a report, then write it! That way all your bases are covered. Remember that all internal investigations usually start with the question, "Was a report taken?"
In a nutshell, handling any call involves figuring out whether it's either civil or criminal. Once you figure that out, each one has its own list of dos and don'ts. Each one also carries with it a specific set of paperwork you must use for agency, state, and federal purposes.
If you can't figure it out, please put your ego aside, place your mission first, and call someone for help. Call a zone partner, your supervisor, or your staff attorney if need be. Make every effort to make the best decision possible in determining what you have.
The start of this process is more important than the end because your first decision is the most impactful. Make the right decision and the world loves you. Make the wrong one and the world tries to swallow you. Letting someone go with a warning for what you thought was a misdemeanor but turned out to be a felony never reads well in the media.