Well, I suppose most English speakers just know from memory. The two endings sound completely different, but other than that they are hard to distinguish. Also, I found this from a website that should be helpful... "As far as determining which words end in -ER and which ones end in -OR, the Oxford English Dictionary makes some historical distinctions, attributing -ER to words of English origin and -OR to words that entered English from Anglo-Norman, Old French or Latin. " (Calworks) This website has more information on the distinction between the two, if you'd like to check it out the web address is: http://calworks.helpscc.org/index.php?option=com_content&view=article&id=271:grammar-grater-er&catid=60:grammar&Itemid=924 Hope this helps!