Whether you are a one spacer or a two spacer, being consistent is probably the most important thing. Here, Microsoft Word can help by checking your sentences to see whether you have one or two spaces after the periods. To turn this on, open up the options menu (file => options in Word 2010) and go to the proofing tab. From there, click on the settings button in the spelling and grammar section. Near the top, you will be able to tell Word whether you prefer one or two spaces after a period. Word will then flag whenever you have either too many or not enough spaces.
As a side note, while you're in that section, you can also tell Word whether you want it to look for the use of the serial comma and whether you like your punctuation inside or outside of quotation marks.