There are a lot of methods of gaining comfort with the site you are using. Reading through the site will provide some measure of a clue. If there are a lot of poor sentences or it sounds like a foreign person put it together or a second grader, then you can bet it's probably not a good choice. I've used TheWritingReliever.com and ScholarlyAdvantage.com and both have been awesome. It's always a toss up when going with a new site. Just be careful and take the time to review the writing on the site. If they have writing samples, it's usually a better chance of quality and legitimacy.