Most probably, you know friends who have used pre-written essays. These are in a better position to advise you on the best services available. Ask them their experience with some of the services you think of contacting. Get views from different friends to determine the most appropriate service for you.
Some of those sites employ non-native speakers and writers without university degrees and that might impact on the quality of their works. To avoid this, read as much custom reviews as possible. You can also check the requirements for applying on the service as a freelance writer.
Custom essay writing services should have 24/7 customer support. You need to communicate with the company to know the progress of your order. Check of all the communication channels are working. A reliable service should respond promptly to your questions.
Some have opined that students go for these services because they are lazy. This is partly true, but the biggest percentage that goes for these services are students who are trying to beat serious deadlines or have a deficit in their writing skills.
In college, a student gets to write too many types of essays. The type of essay writing service you hire must be able to write in good time. If you need an argumentative essay and all they can manage is an expository essay, then they are not the best source of help.