There are a couple of ways to go about this. Ask them point blank if they believe that Our Lord is God. You see, Mormonism teaches that the Father, the Son, and the Holy Ghost are three separate and distinct beings. So if they say that He is God, they either contradict the Mormon teaching, or imply that there is more than one god (which they also teach, though they will quickly say they only give worship to one). If they deny that Jesus is in fact God, they directly contradict Sacred Scripture, and reveal themselves as non-Christian.
Another way to go about it is to question the need for a “restoration” when they teach that St. John the Apostle still walks the earth and will live until the Second Coming. Plus, three “Nephite Apostles,” also to live until then. All of whom had the full authority of the Priesthood according to their concepts. So it would make zero sense to appoint someone else with no priesthood authority whatsoever to rebuild the Church.
That’s just two. I could easily come up with more, being an ex-Mormon myself.