If the scales start to visibly 'pull apart' that's the easiest way to tell just by looking.
Yours looks lovely and healthy to me, but if he piles the weight and you're concerned, on just increase the time bewteen feeds or reduce the amount of food items (if you feed more than one).
Bear in mind that some are just beefy though!! Mine's never been loaf of bread shaped, even when he was a tiddler :\