This is an excellent online dictionary that offers both UK and US spellings.
A free online dictionary. Plug in the word and check . . . and don't forget to check out the thesaurus.
Here is an excellent reference guide for Americans attempting to stick to British spelling. It very clearly shows the differences between American and British English.
Look up words in either an American or British English dictionary. Don't forget to look at the tables and chapters. This site has a wealth of information.