Well the answer to that is with difficulty . Firstly the 1915 is rarer than the 1916 and that most collectors associate the type with the broken tooth and want it to be clearly visable . And so without it, its really hard to tell from a photograph, but in hand its easier to determine the depression in the centre of the coin. Below are four pictures of the tooth gradually looking more broken
Note in the top picture the tooth looks to be intact, but on closer inspection it is actually very slightly smaller . really hard to see on a coin. ?