If you post an example that you are stuck on,
it will be much easier to answer specifically.
Then the general cases will be much more obvious.
However, all terms in both the numerator and denominator are multiplied, so they are all factors.
Do you mean if you had terms combined by addition or subtraction instead of multiplication?
Then you try to factorise if possible.
If you are more specific,
someone can help you with an another example, if you post one.