You said what all of the Baofeng owners were thinking but afraid to say. A lot if Ham owners have the mod and I wish that I would have gotten my Yaesu done before they shipped it out to me.
I suspect that enforcement is "light", because of the difficulty in remotely distinguishing the signal strength difference between a 50W modified ham rig transmitting on GMRS frequencies with a 2dBi gain antenna** from a 15W GMRS rig with a 6dBi gain antenna** when the line of sight is unobstructed and the receiver was in the center of the signal path for both. Once you throw terrain into the mix, it would be even more difficult to distinguish.
With that said, the ham rig could also transmit on frequencies that fall between GMRS channels, and that would be a clear/easy indication that a ham rig was being used instead of a GMRS rig.
**
Nerdy antenna details you probably already know: Think of the antenna gain like this: a 0dBi gain antenna essentially spreads the power of your signal across the entire sky (actually ground too, but the ground largely absorbs the signal at the frequencies we use). Unless there is a receiver directly above you, a lot of that power is wasted. Antennas that provide more focus of the signal toward the horizon are said to have higher gain - the higher the gain, the more focused. A lot of mobile ham operators (and some GMRS operators) own both a high gain antenna ( ~5-6dBi for flat terrain) and a lower gain antenna (~2-3dBi for hilly/steep terrain). Others things being equally, a 2x increase in transmit power achieves roughly the same signal strength at the horizon as a 3dBi increase in antenna gain, and a 4x increase in transmit power achieves roughly the same signal strength as a 6dBi increase in antenna gain. A cheap antenna cable/connectors could lose 2-3dBi before the signal even gets to the antenna. Moral of the story: a good antenna, good antenna mount, and good cabling/connectors without any extra connectors basically doubles the transmit power (and reception capability) of your radio.