Copperheads give off an odor of cucumber when they're threatened. If you suddenly smell cucumber, stop moving and start looking around. My mom has a few in the rock outcropping behind her house. Copperheads are very docile though and will usually try to flee and their bites are usually not fatal to healthy adult humans though one could easily kill your dog.
Cottonmouths are related and they're even less of a threat. They stay in piles of tree litter near wet places and are generally very shy.
I would worry most about rattlers. Some rattlers have quite a temper and may try to hold their ground rather than flee. Just remember that rattlers rattle because they would rather not bite. They can also be hard to see against the rocky shale and granite outcroppings they tend to favor.
The nastiest snakes are the rat snakes. They're happy to strike because they have no precious venom to waste. Though alarming, it's rare they even break the skin. Be aware that rat snakes like to rattle their tails, though they don't have rattles, against dry leaves to make you think they're rattlers and thus hope you will leave them alone.
If you're in an area where you think snakes may be around, make a point to stomp on the ground. Snakes are very sensitive to ground vibration, it's how they hear. Stomping, or at least not trying to walk delicately, tells them something big is coming and to leave the area.
There's not much you can do on the trail when it comes to snakebite. The best thing is what was said, keep the animal or person quiet, call a ranger or EMS as soon as you get a cell signal, and get medical attention ASAP. Viper venom causes tissue necrosis at the site of the bite so it's extremely important to get medical attention immediately because even if the venom may not kill you, it can cause the loss of a limb or organ failure.
Look at a few websites for the various venomous snakes that may live in the area you're visiting. Both copperheads and rattlers have various colorations even within the same species. The general rule is that if the bite is two small vampire-like deep puncture marks accompanied by swelling and tenderness, then it's venomous bite. If the bite makes a horseshoe pattern then it's non-venomous. Rattlers can control the amount of venom they inject and will sometimes not inject enough to kill or even any venom at all. Don't rely on this however; treat any venomous snake bite as life-threatening.