Many dictionaries will give the etymology of the word. For example, the word "origin" has Latin roots. http://dictionary.reference.com/browse/origin

I've noticed that KP has already given some useful advice about prefixes and spelling, so here are a few more clues to identifying Greek/Latin words. - Most of the time, those same words came to us through French, so French spellings like "the silent e" and endings like -ion also give us a clue. - The Latin/Greek words tend to be long, and even though you can divide them into parts, the separate parts don't usually function as separate words. (Unlike longer Anglo-Germanic words, which you can divide and use as separate words.) - You'll see these words used in formal and academic contexts. Using them in regular speech sounds a little awkward, and if it's a verb, we'd normally use a phrasal verb in daily speech instead of its formal equivalent. - Even if you know one or two random words in those languages, if you also recognise those words in English, then it's probably a Latin/Greek origin. For a long time, the borrowing was entirely one-way, into English.