Where do you go to find out what a particular tract of land is zoned for?

In most areas of the US zoning is managed by the local jurisdictional authority...typically the County planning department. Sometimes it will fall under town or city jurisdiction. To identify the zoning of a specific tract of land I would contact your local planning department as they should be able to tell you the zoning, and provide you with the zoning guidelines in that area.

Zoning may also be found via county records and your local title company can pull the records (as they do during real estate sales process) and these records will usually provide the zoning designation (residential, hillside, rural, williamson act, PUD, etc.)