Type into a word document program with automatic correction features. Most document programs have default settings that will automatically pick up on errors and underline them in bright colors, providing you the option to have the mistakes corrected by the computer.
Utilize an online grammar checker. Numerous grammar checkers are online. You simply type in the sentence, and the grammar checker will show you if any corrections need to be made. Some grammar checkers will even explain why something that was missing needed to be used, almost like an English lesson would.
Read the sentence aloud in order to catch any errors. Where the sentence ends, your voice usually drops and a period would be added at this point. Where you naturally pause, there would most likely be a comma. If your voice raises (as in asking a question), a question mark would be appropriate.