I agree with TN.
Take a close look at their web sites. Try to evaluate where the real emphasis is. If they are not selling 'guaranteed' jb placement with the course, they may be OK. If they are not providing courses, and admit to being purely a teacher placement agency, then as long as they are not asking the teachers for money, they are probably OK. If they are offering guaranteed work, free scuba diving lessons, free participation in an all-night beach rave party, free local language lessons, and free local cooking lessons, start getting suspicious.