Reading the Bible? Well...
Reading an introductory book on the subject? - But which one? (All are from the perspective of one denomination or another).
Listening to Christian leaders - The Pope? The Patriarch of Moscow, or Constantinople? Billy Graham? The Archbishop of Canterbury...
(Just kidding about the Abp of C!)
Reading some 'objective' source like Wikipedia?
Observing the daily behaviours of devout Christians - but there is little similarity between a Roman or Orthodox Catholic and a Home Church protestant or the Salvation Army?
Tricky isn't it...
And I don't have any better answer than CS Lewis's Mere Christianity - but of course that is implicitly Protestant.
So... How would someone find-out what Mormons believe?
(My suggested Answer is not to read The Book of Mormon, nor even the Doctine and Covenants; but to spend a couple of hours randomly browsing from among the many hundreds of videos - short and long, mostly for internal-consumption - on:
Then compare what you discover, with the descriptions of "Mormonism" that you find around and about the interweb.
Go ahead - try it!)