Use others. Trying to determine if your own code is readable & maintainable or not is inherently a bad idea. It's in fact paradoxical because if you could do that initially, you wouldn't even be asking this. I'm sure most of the people on here know how mean and critical people online and especially on StackOverflow can be.
Use this to your advantage. Write some sample code in the same style of your regular coding (unless you can share the source) and post it on codereview.stackexchange.com or here at Hashnode.
People will probably slam and critique your code and while you may not agree with everything they say, you will get some take-aways and learn things. Keep doing this over and over.
I'll elaborate a bit on how to handle critiques: Someone may show you a more clear and performant want to do something... This is probably good. But if someone slams you for using tabs rather than spaces, or something trivial like your comment style, then you can evaluate yourself whether or not the feedback should be implemented or just noted.
This is also assuming you don't have direct access to reputable coders for review in person. Do both.