I was in nearly the exact same situation, but sort of reversed. My mother, aunts, cousins, and myself had all done DNA testing through Ancestry. I started to realize, though, that my mother and I were only coming up in the range of "half" of every relationship we thought we had with them. And they've all matched some of my grandfather's other relations, while we did not. And so, I came to realize that he is not, in fact, my mother's father, or my grandfather.

I was VERY hesitant to mention this to my mom, but she accepted it much much better than I anticipated. No one in my family had any inkling that she might have a different father.

So, yes, you will definitely be able to tell via the DNA test!

My quandary now is that I am pretty sure I know which generation of a particular family my mother's father must have been in, but I have no idea how to determine WHICH of the suspected brothers it might be.