This excellent article rewriter tool will thoroughly scan your entire content, looking for the words that it can replace with a synonym. All the words that it replaces with a substitute will be highlighted in the bold and colorful text. By clicking on any of the highlighted words or phrases, you can see the original word, suggestions or add your own word. Also, you can click on the original word to revert or click on any other word to replace.
Never let your audience guess what you are trying to say, your writing should literally speak for itself. When your content has too many errors, it becomes difficult to read and understand. Although, one or two errors can be skipped by readers, but glaring grammatical errors can minimize the impact of your content. Also, it can make your writing difficult to read and digest.
Imo it is not possible for google to detect properly spun content, it is not worth computing power it requires. They can do this using ngrams for example, but it is uses a lot of queries. And for example article spinner I coded beats this algo, so google cant detect that the content is spun. Manual spinning is also good but it takes more than 30 minutes per article.