If the material has broken down, it may absorb fuel over time. Observe the float in a small container of gas and note (or take a picture) of how it floats. Submerge it with a weight overnight and then observe it without the weight again. If there is no change, should be good to go.
If you're talking about actual plastic floats that are hollow, that's different. I would warm them up pretty good and then submerge them (opposite of the brass test). As they cool, they will pull in gas and you'll hear it sloshing around.
Or...go to Mike's carburetor parts and grab some new ones.