How do you spell difficult words?

If you are writing an important paper, and you need to spell a word but are unsure of it’s correct spelling, what do you do? Do you risk guessing and possibly getting it wrong? If you are a student, you know that could possibly lead to a bad grade; and if you are a journalist, copywriter or other working professional, it could lead to your work not being seen in the best possible light.
So, how do you spell a word? You could take the time to look up every word you are uncertain of, OR you could use the online spell check. Simply cut and paste your document online, and the spellchecker will automatically tell you which words are misspelled and give you alternatives. By using the spell check, you eliminate the possibility of having your work look unprofessional, or careless by having misspelled words.
So, if you want to have professional looking documents, how do you spell check?