If it's valve guides/seals the usual symptoms are smoking on the overrun or a puff of smoke after opening the throttle following an overrun situation.
If it's rings is usually smokes the whole time, but there is only one way to find out for sure - open her up and check everything.
Another thing that can cause smoking is borewash, when the fuel mixture is so rich it washes the oil of the bore and then burns it as part of the combustion process, left unchecked it will cause premture wear.