My 'Shorter' (2 volumes) Oxford Dictionary uses -ize, which is useful for checking the words that have to use -ise. These are usually those ending with -vise, words to do with seeing, such as supervise and televise, or -cise, words to do with cutting, such as incise.
I generally find the Longmans dictionaries useful for pointing out UK/US variations.
What is important is to be consistent. For any piece of writing, where a word can be spelled in more than one way, decide which spelling you are going to use, and stick to it.