Tipping a caterer 15-20% of the total service charges is good, though not exactly written in stone, guideline. In some cases, it will be less depending on if the party catered was a huge one.
In other words, you can expect to pay about $20 for a nice little dinner for four people and around $60 for a wedding reception with 250 guests. You should figure out what percentage to tip based on your bill.