Microsoft Word usually does a good job, you specify the proofing language (US, British, Australian, Jamaican English etc.) and it highlights the 'mistakes'.

You can think of American English and British (UK) English as dialects where the words are the same but the SPELLING is different. You will find people from Australia will usually use UK English with a little bit of US English, while people from Canada will use a combination of US and UK English. UK English will usually be the one with more vowels (for example UK:Colour/US:Color, Aeroplane/Airplane, Caramel/Carmel) or more double-Ls (for example Traveller/Traveler, or Marvellous/Marvelous). US English is usually spelled more like the way it sounds (for example US:Capitalize/UK:Capitalise, or Defense/Defence). This isn't every difference and there will be many confusing exceptions. Here is a link that might help you: http://www.tysto.com/uk-us-spelling-list.html

You can use an online dictonary such as the Oxford Dictonary, where they tell you if a word has a different variant in British English or American English. The majority of the words are the same for both countries, you'll need to worry mostly about specific words o sometimes technical ones, and about slang terms.

Generally, the main difference is in the spelling of words; in other words, American and British English sometimes have different spellings for words. However, there are some differences in words/meanings too. Watching American/British TV may help you figure out the differences between them.

Thank you, Paul. I'll remember that.