My experience has been pretty much the same as the previous posters. I made the mistake of checking the box showing I had been on a farm outside the US. I was on a business trip through Thailand, Malaysia and China scoping out some new products and it entailed tromping through a lot of farm fields in these countries.
My compatriots didn’t check the box and whizzed through Customs. My shoes got the once over bigtime but I don’t remember anybody poking the treads with a tool. Plus I got the 21 Questions routine. Slowed me down maybe 15 minutes so no big deal I guess.
But this thing is purely on the honor system. There is no way they know if you were on a farm unless you decide to be honest about it.