Peg teeth are much smaller than an incisor and placed a little further back. If it is only just poking through then it may be hard to tell to start with but will become more obvious as it gets bigger. The peg tooth will be much narrower than an incisor.
Poor Hermans peg teeth have grown back 5 times now so we have left them alone. His don't get over grown like incisors do and cause him no bother