Hard to say. Other than x-rays of your wrist there is no way to tell. And, you don't need to. You will stop when you do.
Depends on bone age. The potential additional length anyone can expect depends on areas in the long bones (arms, legs, fingers)called growth plates. This is where growth occurs and an active open growth plate means more growth is possible. An x-ray of the hands & wrists can be evaluated for bone maturity since there are so many to see. If your bone maturity is average for a 18 yo you are done growing taller.
Maybe already? Most 18 year olds, even late bloomers, have already finished growing. If you want to know for certain, doctors can tell you more precisely by doing an xray (usually of your left hand) and seeing if your growth plates are still open.