Second the Seoul Medical Center in Bonghwasan - soooooo efficient and everyone was very kind. It's about an hour subway ride from where I live - but so worth the trip.
[Edit: Their English was fine - there's usually a interpreter at the information desk and the nurses know enough so you know where you have to go and what to do, etc. Just don't forget the two passport size photos! They won't do the check without them! There's a Homeplus nearby where you can get them done - go to the photo counter and ask them there - it's about 9,000 won and a 30 minute wait. Also - they stop doing health checks at 4pm (but none on Saturday - I think?), so be sure to get there in time/early!]