Microsoft's grammar checker isn't great, but it's something. Other than just knowing the language from either education or immersion, you basically need a native or fluent speaker. I'm sorry there's no easier way that I can think of.
I searched google for the pattern: online grammar checker I tried several of the search results. One looked like a scam. Another one did a very bad job of correcting mistakes, but this one actually works quite well: www.gingersoftware.com/grammarcheck