To identify a green caterpillar, make note of its shade of green, main body pattern, hair density and any unique features. Refer to a website or reference guide on different types of green caterpillars, and compare the characteristics of different green caterpillar species to the characteristics you noted.
The big green caterpillar is classified as the lunar moth caterpillar. Caterpillars with white stripes down their bodies include the pellucid hawk and the angle shades caterpillar. Hairy caterpillars include the pale tussock and the saddleback caterpillar. Green caterpillars with black spots on their bodies include the snowberry clearwing and sawfly, while the old world swallowtail features yellow spots. There are also caterpillars that have horns, such as the tobacco hornworm and the tomato hornworm.
The saddleback caterpillar and angle shades caterpillar are two caterpillars that are best not to disturb. The bristles on the back of a saddleback caterpillar emit a potent stinging chemical, whereas the angle shades caterpillar lets out a foul odor when it gets disturbed. The puss caterpillar is another caterpillar to avoid, as it can cause your fingers to burn and swell if you touch it.
There are several websites that offer species identification guides, such as the one on DiscoverLife.org. ButterfliesandMoths.org provided links to sites that have identification tools catered to specific regions.