By comparing them to NOTHING!!!!
During his researches and schooling, however, he came to realize the original words were long lost and it would be difficult to recreate them from all the different versions of Scripture that have come down through the ages ...
Idiot. He compares them to each other. He and others like him reviewed hundreds and thousands of manuscripts from the very beginnings of Christianity through the early Middle Ages. That you so blythely dismiss the work without reading it speaks poorly of you.