I think its partly things like watching shows you're familiar with in your native tongue- I know, for instance, that I could watch all of the 3 Stooges stuff in Hungarian. Ditto Gilligan's Island.
And I watch enough of shows like that, I'm going to be able to pick up on certain linguistic substitutions. Take GI for instance...by my second episode- heck, possibly by the second commercial break- I will probably have figured out the Hungarian words for "Professor" "Skipper" and "coconut", maybe even "minnow" and "storm"
Then there's that whole "common word" and "visual cues" thing.
If you are standing next to a Hungarian having a phone conversation, and you hear him repeating a word while nodding his head, you'll figure out pretty soon that "egen" means "yes."
Or you'll see a "No Smoking", "Police", "Stop" or "Danger!" sign and figure that one out.
Do it long enough...