Most modern word processing software (and also many other types of application as well) has a built-in spell checker to confirm that each word you enter is in the computer's dictionary.  However this type of accuracy check will never tell you if you have typed an incorrect word.  For example look at the following poem:

Spelling Checker

Eye halve a spelling chequer
It came with my pea sea
It plainly marques four my revue
Miss steaks eye kin knot sea.
Eye strike a key and type a word
And weight four it two say
Weather eye am wrong oar write
It shows me strait a weigh.
As soon as a mist ache is maid It nose bee fore two long
And eye can put the error rite Its rare lea ever wrong.
Eye have run this poem threw it I am shore your pleased two no
Its letter perfect awl the weigh
My chequer tolled me sew.

--Sauce Unknown

Every word in the poem is correctly spelt - but almost every one is wrongly used!  

Grammar checking

Also a feature of most word processing applications - grammar checking is useful - but may suggest changes that don't need to be made.  for example Word for Windows always tries to change sentences like this one:

This is the joke which I told last night.

to this:

This is the joke that I told last night.