How to verify that code you write is not copyrighted anywhere? Is there any guidelines how to make sure that code you write won't be copyrighted anywhere? I meet sometimes situations where I write code that does some particular small thing but there is a similar code on the Internet without licenses. Then I need to think alternative more unnatural solution or how to modify the code more messier to avoid copyright issues.
Don't worry about "similar" code. Copyright is about copying, not about similarity. If you wrote it independently of any other, then there shouldn't be an issue. 

But there is also the rule that in some situations there is "only one way" to say something. So if some phrases (or coding patterns) appear in several places it isn't evidence of infringement. The number of ways you can write an iteration through a list is pretty small, after all. Problems recur in computing. So the "natural" programs that solve them also recur. 

Also, copyright is about publishing. If you don't publish your code then it won't be an issue either.