Most modern word processors come with a spell-check and grammar checker program. Trying to edit off the computer screen is almost never accurate enough. After correcting all the errors you found the first time around, spell-check again. Print it out once more. You must define these situations so the reader doesn't become confused.
Viewpoint is important.
In contemporary writing, it is most correct to skip a couple spaces when changing viewpoint, just as if a scene change is taking place. Some authors change viewpoint paragraph to paragraph, but this is a difficult technique to master, and, if done incorrectly, negates the pleasure of reading your work. Again, input all necessary changes. At this point you may even need to re-write certain areas for clarity and such. Do yet another spell-check.