Batteries have an electrolyte solution inside them. If that solution is frozen, don't try to jump-start the car. If your battery has removable vent caps, you can look inside to see if the liquid is frozen (replace the caps after looking). It's not easy to tell otherwise.

• If there are cracks in the battery casing, don't try to jump-start it. Curse a few times, call a cab and go buy a new one.
• If there is whitish (or greenish or yellowish) residue around the battery terminals, clean it off with a wire brush. Wear gloves, if possible, because that stuff can be nasty if it touches your skin. When this stuff forms, it's a sign that you may need a new battery.