Asking my critique partners for help with my grammar and punctuation has been what works for me. Sometimes I'll ask if they can specifically point out any errors. Since I struggle in those areas, it's nice to have partners who are knowledgeable in them.
Also, lately I've been using the website quillbot. So far it's been helpful with my grammar and punctuation. Some of the suggestions aren't always accurate, but for the most part it's been helpful. There are many other free grammar sites you could try.