Muslims cant prove ISLAM is right to Chrisitian, because our entire case is based on whether God actually spoke to the prophet… Now we cant prove that to Christian useing the Bible.
In the same way, the Ahmdis say that God himself declared their faith to be true. Now you cant prove god didnt speak to them…
And so if you say they cant prove anythign according to the Koran, they will say you had misunderstood the Koran to begin with because god said so…
Basically its a mtter of proving what cant be proven.
So you might aswell get used to it. You have as much credibility in assuming your version of Islam is right as they do in assuming theirs is.