You could google the School and see what other teachers may have posted after working there. This webpage has a School's Review page. The link for it is at the top left corner of the screen. The best way to know if ILS is a good School, and they are few and far between in Korea, is to speak to foreign teachers already at the School, and not teachers involved in recruiting for the School. If the School is not willing to let you talk to its foreign teachers, know that something is wrong. Scan the contract carefully. In South Korea, a respectable hagwon will enroll you in the pension and medical insurance plans, and offer vacation and sick leave. It will offer you a cell phone and a well furnished apartment. It will have some sort of student testing and grading system. "Just have fun" is not a good sign.
Find out if they will loan you to other Schools. If so, will they register you with Immigration that way. If not, stay away from them.
Lastly, why would you want to work in South Korea anyway?