Yes, there are words in English that sound similar, but are in fact not related at all, and the similarity is just a coincidence. Other times, of course, similarity is because one word is derived from the other or they have a common root.
I don't know what your native language is. But are there not words that sound similar but have no relation to each other?
I have difficulty imagining how you could tell just from looking at two words whether similarity is significant or coincidental. Perhaps linguists have researched types of similarities and patterns and have developed rules, but I'd be hard pressed to think of any way you could look at "aver" and "aversion" and discern that the similarity was coincidental without looking up their definitions and etymologies.
So short answer: There's no easy answer. You have to know the words or look them up.