Did the kla'yim enforcers actually check all the fields in the country? Sounds like a massive effort, did they get paid for their work and how were they selected for this job?
Pinchos Osher, Jerusalem
It seems that they would check any agricultural area, but perhaps they concentrated their efforts on the "problem" areas where the owners were more suspect of overlooking the Halachah. Yes, it was probably a big effort.
The Kil'ayim police were paid with Shekalim of the Terumas ha'Lishkah (Rashi and Rishonim Moed Katan 6a), as were all the others listed here who were hired for public services rendered by Beis Din. That is why the police were sent (the second time) during Chol ha'Moed (Moed Katan ibid.). Since no other work was available at that time, labor was cheap and would not overburden the public coffers. Presumably, poor people would jump at the opportunity. There probably also were specially trained, brawny police, chosen to fearlessly uproot Kil'ayim from the largest of fields.
Best regards,
Mordecai Kornfeld
Kollel Iyun Hadaf