Upvote:3
Maybe it varies between airports, but where I have used PreCheck, it works as follows.
At the entrance to the security checkpoint, there is an agent who I'll call the "greeter". You show your boarding pass to the greeter, and if they see the check mark, they wave you into the PreCheck line.
But when you reach the front of the line, there's another agent at a podium with a barcode scanning terminal. You hand them your ID (driver's license, passport, etc) and scan the barcode on your boarding pass. I do not know for sure, but I would assume that the barcode encodes your name and PreCheck status, along with a cryptographic signature, or at least a link to a database with this information. This information is displayed to the agent on their terminal.
So if you were to take a non-PreCheck boarding pass and forge it by adding a check mark, you would get past the greeter, but all that would achieve is letting you stand in the wrong line. When you got to the ID check, the barcode would show that in fact you do not have PreCheck. Best case, you explain it away as some sort of mistake, and they send you back to the regular security line. Worst case, you get arrested.
Something similar would happen if you tried to take the boarding pass from an actual PreCheck passenger and change the name - they would see the name didn't match your ID.
The greeter is more for efficiency than security - they mostly just keep people from wasting their time by standing in the wrong line by mistake. They are more visible because you see them actually looking for the check mark on a boarding pass, but they are not really the functional part of the process.