There are a lot of different programs out there that will check for spelling and grammar errors. Most word processors have spelling and grammar check functions built into them, and running these a few times will help to catch basic errors. Reading your cover letter out loud will also allow you to pick up on errors as well as identify any spots where the sentence flow isn’t natural or where the meaning of what you wrote isn’t clear.
It is best to consult an expert to get a more holistic review of your cover letter to ensure it is error-free and that you have pitched yourself in the best way possible.