Play Pinkerton games, win Pinkerton prizes.
Or, less flippantly: everyone knows who the Pinkertons are and what they do. And the fact that everyone knows is, quite literally, the actual service that they offer. They will do such things without you have to explicitly tell them.
Movies and TV often sow them as a policing the West in the early days of the US but what they should be known for is as thugs and strike breakers for hire who intimidate and threaten whoever you ask while providing distance and cover.
Aside from the xkcd, this 'everybody knows' effect can be domain specific. The people who hired the pinkertons, and any party interested in stealing corporate secrets, know who they are.