How do you know That McGinty is heading a criminal organization?

From the text:

McGinty was the name—Bodymaster McGinty. I asked him when he recovered who Bodymaster McGinty was, and whose body he was master of. ‘Never of mine, thank God!’ he answered with a laugh, and that was all I could get
from him. But there is a connection between Bodymaster McGinty and the Valley of Fear.”

If you’re in trouble in Vermissa, go straight to the Union House and see Boss McGinty. He is the Bodymaster of Vermissa Lodge, and nothing can happen in these parts unless Black Jack McGinty wants it.

“If killing is murder, then God knows there is murder and to spare. But don’t you dare to breathe the name of Jack McGinty in connection with it, stranger; for every whisper goes back to him, and he is not one that is likely to let it
pass."