The only real rule is there aren't any rules.

I use what I call a clock box. A wooden box with a rock weighted lid. A big rock so it takes a big hog to move it. A couple of handfuls of corn around the box and a few handfuls in the box. Copper strips make contact when the lid is on and the cheap electric clock runs. I sometimes wet down the earth around the box a bit.

In other words, I time when the Hogs tend to show up and read the tracks to see who shows up. Old school, but it works. If somebody (earth first types, plain old vandals or dope growers) smashes it or steals my clock, I haven't lost much.