It's hard for me to imagine that they have an army of radio engineers to go looking at all signals, figuring out which ones are "undecipherable", physically identifying the transmit locations and then determining who did it.
The entire culture works hard to weed out anybody that doesn't follow the rules. If you do something you're not supposed to, there's a good chance someone will figure it out and report you.
Foxhunting is a sport for them.
Its especially common these days with the advent of software defined radio, spectral displays and large amounts of storage for operators to come across a strange transmission and record it for further analysis, often posting in forums and sending the recordings to other people for them to try and analyze it.
And some times those recordings are sent to the FCC and the FCC sends out strongly worded letters.
If it really gets bad, the FCC will act and IIRC they average about 20~50 or so fines a year.