If you've got a volt meter, check your battery voltage with the engine and all accessories off. It should be around 12.6 volts. Once you start the engine (again, all accessories off) it should jump up to the 13-15 volt range. There is more to it than this simple check, but it's easy and a good place to start. If you don't have a volt meter, you can get a cheap one at Harbor Freight for like five bucks.

'91 LWB, 31x10.5s, KYBs, A/T "I look like Robert De Niro, I drive a Mitsubishi Pajero" - Billy Bragg (not the original lyrics, but that's how he sang it when I saw him in Dublin in '04)