Just play. That's about the only advice I can give you. You'll eventually learn what characters you need to block high and low against.
Or, you can look at every characters moves in practice mode and see which ones start out with unseeable lows and overheads. For example, Batman's b1 in an unseeable low. So you should always be blocking low if he runs up to you. His other strings that have overheads in them don't start with the overhead. So block low and if you see them do a string that has an overhead in it, start blocking high. It all just comes with experience.