How do you know when your guinea pigs teeth need to be clipped?

ive had mine for a year, and ive noticed he hasnt been eating like he used to. It might be because of the new dry food i gave it, but i give him the same timothy hay, and before he would eat all of it as soon as i put it in there, but now ive noticed he eats a little and walks away. Does he need his teeth to be clipped? Or could there be something else wrong with him?