Here's what you need to know in advance about how to harvest roadkill.
Not everyone resides in states where it's legal to harvest roadkill, so make sure that you are legally able to do so BEFORE shoving an animal into the back of your truck. Standard IANAL thing here. You have to do your homework on that account.
That said, if you are able to legally harvest roadkill, there are a few things to know before doing so.
Check for Impact Damage
First and foremost is to look at impact damage. Most people harvest roadkill because they want free meat and not a taxidermied squirrel army.
This is important. Obvious tire marks and impact on the body? That probably indicates a certain amount of gut spillage. Mangled hind or forequarters may mean that meat may not be salvageable.
Gut spillage isn't the end of the world, as most prime cuts of venison aren't located in the body cavity and - in the right conditions - can be cleaned with some effort. However, you don't want to take too many chances. So, make sure you have a look at the body before attempting to salvage any meat.
Do the Wild Game Processing at Home
Wild game processing should be done at home. State law may even mandate that you do so. (The recent Washington roadkill law, for instance, does just that.) Spending time on the side of the road is dangerous; doing so when you don't have to and are legally obligated not to be doing so is just stupid.
Fresh Kills Only, Please
The only roadkill worth keeping for survival or free meat for the freezer is meat that hasn't had the time to spoil. The carcass has to be butchered before rigor mortis sets in.
How to gauge how long an animal's been dead?
First, according to Survival Sherpa, look at any blood. Has it congealed or gone dark in color? Fresh blood is bright and lacks viscosity. The darker and thicker the blood, the longer the animal has been dead.
Look at the eyes. Are they clear, or cloudy? The eyes go cloudy within a few hours of death. If the eyes are clear, keep the deer.
Pinch the skin. Does it freely move? Or does the skin stick to the muscle? If the latter, rigor has set in and you should probably steer clear. If hair pulls away with ease, then forget it.
There are also certain insect indicators. Fleas are a good sign, since that means an animal is still warm. Flies, however, are not.
Once you have the animal home and have begun processing, examine the internal organs. If they appear off-colored, exhibit a yellowish-greenish discharge or smell putrid, you should dispose of the carcass. That can indicate either an infection and/or rotting has begun. In either case, it's wise to dispose of the carcass and not risk eating tainted meat.
A Strong Smell Means Stay Away
Duh. A strong, putrid odor means it's rotting.
Asphalt Hunting Is Best In Winter
The higher the temperature, the quicker meat spoils, so any asphalt hunting (as it's sometimes called) is best done in the cold months. The cold stalls or slows down the decomposition process. If it gets cold enough, a carcass can actually freeze.
However, that doesn't mean a frost-bitten animal is safe. It could have started to rot before it froze.
That said, a carcass is safer for longer in cold conditions. Make sure to thoroughly inspect any roadkill before removing it. During the warmer months, it's not advisable to harvest a roadside kill unless it's visibly very fresh or you literally watched the animal die.