If the problem is on the outer surface of a tooth you may be able to feel it just running your hands along the outside of her cheeks - look for differences between one side and the other.
Otherwise you want to get inside her mouth - in a cow you'd pull the tongue to the side and flip it over, then you can inspect the tongue for sore spots caused by rough teeth, but I don't know that that would work in a sheep... If you're sure of your grip, you know they only have bottom teeth - no upper teeth - at the front, and a no-tooth gap behind that, so you can hold the lower and upper apart...