"Side effects", or "Side-effects"? [duplicate]
Solution 1:
A great resource open to you for looking at the frequency of words or phrases is Google Ngram Viewer.
As Brian Donovan said in the comments, this Ngrams analysis answers the question quite well:
As this shows, side effect written as two words has been a lot more common throughout the past half-century with the hyphenated version used much less. That said, the recorded uses of side-effect are not negligible, some people do hyphenate.