American English is slowly seeping into the UK. This is in no small part down to the boom of US TV, media and novels over the last few years. British English tends to be the norm for formal and academic documents, so it's good to be able to spot the imposter. Here are four key discrepancies between the dialects (US variants are first):
Encyclopedia - Encyclopaedia - The historic dipthong 'ae' still remains in British English spelling but has largely been simplified in US English to 'a'. Medieval / Mediaeval has a similar difference.
Center - Centre - Another common switch is to change the position of letters "-re" at the end of the word. See also metre.
Color - Colour - Most words ending in an unstressed "-our" sound in UK language become "-or" in US spelling. Other examples are flavor and honor.
Dialog - Dialogue - US English has largely eliminated silent letters in spelling for reasons of simplicity. This difference can also be seen in the word analogue (analog in US English)
And here is one point of grammar:
US English does not tend to recognise hyphenated words. So whereas in British English we would say counter-attack, in the US it becomes just counterattack.