Most dictionaries include both spellings and will make note of the two spellings. Usually the differences are noted in one of two ways:
In many US dictionaries the difference is included in the headword with the variation in parentheses:
For example colo(u)r
In British Dictionaries, the Dictionary makes note of the difference in brackets immediately after the headword:
For example watch, plural watches; glass, plural glasses
The table provides a list of the main differences between British and American spelling:
British
-our (honour)
-re (centre)
-ogue (dialogue)
-ence (defence)
-ise 1 (recognise)
American
-or (honor)
-er (center)
-og (dialog)
-ense (defense)
-ize (recognize)
American English spelling sometimes does not double the consonant at the end of a word, while British English spelling does, especially when the consonant is an ‘l’.
For example travel, traveller, travelling (British) and travel, traveler, traveling (American)