Listed below is the most basic of criteria a professional will adhere too.

01: PAT - Electrical safety - Does your DJ have tested equipment?
02: PLI - Ensure your DJ has adequate insurance.
03: Written confirmation and agreements.
04: Experience in the field you are booking i.e. weddings.Children's Parties, 
05: Equipment - Is it adequate for your venue and of a professional standard?
06: Check for specialist music selections for your event.
07: Dress code – Are they able to cater for black tie events?
08: CRB Certificate - Have they been vetted to work with children?
09: Do they have a backup DJ?
10: Do they take requests?
11: Will they play the music at a reasonable volume?
12: Can they be contacted during office hours?

Not all DJ’s are full time but this doesn’t make them any less professional and don’t use this to judge them by.