Westlaw appear to be introducing (on teh scottsih cases at least) a system of flags. If you see a red one it means it is no longer good law and should not be used to support a legal argument - you may need to refer to it breifly though when setting out the new judgement (depening on what your task is). Green means that it is still good law and can be used to support a legal argument.
If the flags are not there then generally there will be, as said in an earlier post, case histories showing where the case hs been overruled, distingusihed or cited.
If you're using paper sources, then it i just a case of following the paper trail until you can find nothing that overrules it.
As for finding cases, statutes etc. you should be shown how to do this by your lecturers, tutors or librarian or all three (both electronically and non-electronically). These are essential skills for any student undertaking the study of Law (we do it in a module called Legal Methods).