Should I always look first in the 98 Report, and only if I don't find something there then look in the User's Guide? Should I always look through both, lest something in the 98 Report be obsolete? Is nothing in it obsolete?
Usually extensions will be called out with a pragma, though, so if there are not {-# LANGUAGE #-} sections at the top then you probably don't need to check the GHC manual. If there are, look over the sections of the GHC manual relating to those LANGUAGE pragmas.