The knee is apart of the leg

This piece of glass seems to be (a) part of a lamp

..

There is not a fixed rule you can learn here.  You have to find a good dictionary and check the different meanings of the same word.  Also, it's good to read example sentences. Sometimes countable and the uncountable words can have very similar meanings (e.g. work, experience) and sometimes not (e.g. iron).

You'll need to give us examples.