In Florida, we have to pass two tests - a core pesticide cert, and one or more of several specific certs. I test for natural areas and weed management, but there are others like l and o, forestry, aquatic, etc. The core is easy but the others are moderately to very hard. Look into your state specific.
I lectured for the Southern Chapter ISA a few years ago on a topic regarding Pest Control Licensing and spoke with Pesticide Division Personnel in all the states encompassed by that chapter as I prepared my presentation. My take away from my conversations with them, was that it's a lot harder to get certified as a PCO in Florida than the other nearby states. The best information resource will probably be your local extension agent.