Grammarly is an Online Tool that claims to “reviews and improves your text, correcting grammar, spelling, word choice and style mistakes with unmatched accuracy.” There is a free trial even if it will be a bit pricey after that. I am doing the free trial right now myself. starting a free trial.
This program picks up not only subject-verb agreement, but also dangling modifiers and better word choices. One thing that has become abundantly clear to me is that our written English ha become quite sloppy over the last hundred years or so. As I tried this program last night, I could imagine my Grandmother, a teacher long ago, smiling from ear-to-ear. "Thank goodness grammar is back in vogue," she'd say.