It's important to keep in mind how much current you're drawing from your vehicle, and to keep in mind what accessories you have that use electricity.
A typical lighting setup (Type-X 8.5" Driving Lights, and an ST3303 21.5" light bar) can draw 30 amps or so, but keep in mind the lights dont draw any current at all when they are turned off.
80-90 amps is a pretty common alternator, so that's often the number you need to stay beneath (although you will need to check on your specific vehicle) as a rough guide, consider that a fridge in the back might draw around 2 amps, your tail lights might be another 1-2 amps, 2-3 amps for headlights, a couple amps for the radio (more if you have a sub woofer and amp) also things like charging mobile phones, UHF radio's, and work lights can add up.
If your driving lights or other accessories put your current consumption over the amount of current the alternator produces, the difference will be made up from your battery (or batteries, if you have a duel battery setup). This isn't a problem, as typically your battery can handle this if it's in short bursts. But driving for a long time consuming more current than is being made will end up in your battery going flat while driving, potentially leaving you stranded.
If you're worried about this happening, you could monitor your battery voltage with a vehicle voltmeter.
In most cases, you wont need to worry about this if adding a normal amount of lights. All of our products show their power consumption in amps on their product pages for your reference. A DC Clamp meter can be used on the battery + wire to see how much current is being used at any time if you want to keep an eye on how much current you're pulling.