This is a moth larva/caterpillar.
I only know this through experience.
In this picture it is eating my Desert Rose, but it is commonly called the "oleander caterpillar" (Syntomeida epilais) and will eat many other plants in South Florida too. It is often found on oleanders (duh). It resembles the caterpillars of the Gulf Fritillary (Agraulis vanillae) butterfly, so many people confuse the two. One difference is that the moth caterpillar has bristles instead of singular spines down its back. Both caterpillars (the Gulf Fritillary and the Oleander Moth) are orange with black (painful looking) pokey parts so it can be confusing. Oddly, neither can sting.
Here is a photo of a Gulf Fritillary caterpillar for comparison.
How else can you tell the difference? Mostly by knowing the plant it is feeding on. Butterflies are often very specific and will only eat the leaves of one genus or species. Moths, however, are typically able to eat all sorts of plants (which is how caterpillars get a bad reputation). Since I know that Gulf Fritillaries don't eat Desert Roses, I know that the caterpillar eating it is not a juvenile Gulf Fritillary.
Scientifically speaking, there is no true difference between butterflies and moths. Basically, butterflies are day flying moths. They are both in the order Lepidoptera (scaly winged insects).
There are some general differences between butterflies and moths, but none that hold true across the board. Typically moths have larvae that can eat many different plants, are nocturnal, have thicker bodies and more "hair" scales (this is because they fly at night when it's cold), have feathery antennae and rest with their wings open rather than folded above their thorax. Yet, there are ALWAYS exceptions. Since moths fly at night, their wings aren't usually as colorful as butterflies'. Since moths are nocturnal, they often drink nectar from white flowers which are fragrant and easier to see in the dark.
All caterpillars are always juveniles and they grow up to be either butterflies or moths (not beetles or mosquitoes or ants).
Be careful if you find a spiny caterpillar.Some have venomous spikes or hairs and can sting. If you aren't completely sure what species it is, you should not handle it. The plant it is eating (if you are lucky enough to find it doing so) is a strong clue to help you identify what kind it is because certain kinds eat certain plants. It helps to know about plants if you want to study butterflies.
That Oleander Caterpillar grows up to be a polka-dot wasp moth (it's actually a moth but looks like a wasp). They are beautiful and harmless. One is pictured below.
Another commonly misidentified species is the Cabbage White Butterfly (Pieris rapae). People often call it the "cabbage moth" but it's actually a day-flying butterfly. This species isn't usually in south Florida but it is very common in most of the rest of the continental U.S.. People are familiar with it because it eats cruciferous vegetables like kale and other garden plants that humans grow for food. Generally people don't like it and so they assume it's a moth. There can be dozens of green caterpillars on your cabbage and if so, this lovely white butterfly is probably the culprit.
Identifying the plants that a caterpillar is eating will go a long way in butterfly or moth identification.