A great way to tell is to yank spark plugs - if they're oily AT ALL you are burning oil and the most likely cause is worn rings and/or bores. It can result from oil being overy thin too - if you check your oil level and it seems thin, or if it smells heavily of gas when you open the oil filler cap (or when you drain the oil, it looks thin/smells), gas is dilluting your oil causing too much to bypass rings and burn (oil dilution is caused by super rich running). When the smoke becomes as heavy as you're describing, it's usually time for a rebuild, or at the very least, new rings. If it's definetly coming out of the exhaust, oil is undoubtedly entering your combustion chambers.