I'm just reading ahead for my DE class, and there isn't really an answer for when the integrating factor can't be found in terms of purely $x$ or purely $y$:
http://www.sosmath.com/diffeq/first/intfactor/intfactor.html
What sort of techniques are there to handle this? Please suggest edits if the question is too broad rather than voting to close (I am new to DE so not sure how to ask the question)