It is know that the FAA conducts ramp checks at airports. However, is it likely to be caught flying without a license from uncontrolled airports or grass strips? If so, how would one be caught?
If they are conducting a ramp check and you don’t have a license, the chances of getting caught are 100%. The size of the airport, the presence of a control tower, or the surface of the runway is immaterial - if it is a public airport the FAA could be there.
If they are not conducting ramp checks, you would only be caught if there were a reportable incident that brings your activity to their attention. (airspace violation, noise complaint or tip, mishap, etc.) Otherwise, how would they know?
I’m not sure how it might work at a private airport, but I would imagine the FAA would be held to the same standards as law enforcement; i.e. they would require either consent, probable cause, or a warrant to enter your property.