It's often hard for foreigners in Korea to check movie times. We don't have any newspapers that publish movie info, as far as I've found. Until I discovered these websites, I was always just forced to show up to the theater and hope for the best. If you use Google search to try to find movie theaters like CGV or Cinus, they just won't show up. Period. In these cases, you need to think like a Korean. Where do Koreans go to find things? naver.com. The Korean Google/Yahoo or whatever. So, all I had to do on Naver, was just type in CGV or Cinus and the first link will take you straight to their website.
Now, the second challenge. The site has no English version. After some trial and error, I found my way to the movie times pages on both these sites. All you have to do is be able to read the Korean hangul to figure out the theater and the movie title. So, for those of you who can read Hangul (and if you can't, the why aren't you getting on that? It's so easy and it would make your time in Korea so much less of a hassle), then these sites are your new best friend.
http://www.cinus.co.kr/Tickets/Ticket_Schedule.asp - Cinus
Some help for you:
Make sure you're looking in Seoul: 서울
Oh, and incidentally, I read in the Korea Times that within the next few months, two theaters in Seoul will be showing subtitled Korean movies more consistently. They plan to release about 10 movies this year with English subtitles. YAYAYAYAY. More updates on this when I know more. I love Korean cinema.