How could one determine whether the gas was a result of decomposition by microorganisms inside the carcass of an animal appeared to be inflated with gas on the road?
An animal on the side of the road that appeared to be inflated with gas
Interesting question! The bloating of a dead body is caused primarily by the activity of the gut flora, the microbes that live in the intestinal tract of all animals. The gut microbes (the majority of which are bacteria) feed on food the host has eaten, and produce methane and small amounts of some other gasses as a byproduct of their metabolism. Normally the host animal's intestinal movements will psuh this gas along through the digestive tract so it can be released as flatulence. When the gut flora continue their life processes even though the host has died, the gasses build up in the intestines and the corpse bloats. Since methane is the main gas produced by gut flora, obtainng a sample of the gas from the corpse and testing to see if it contains methane would determine that it did indeed come from the gut microbes.
There are methane detection devices available, but if you do not have one, you could do a "sniff test", since methane has a characteristic odor. You could also test it by trying to ignite a small amount of the gas, since methane burns very well.