#1. Charge the battery.
#2. Start the engine and measure the alternator output with a multimeter to determine whether or not it is the culprit.
#3. If the alternator isn't to blame, it's time to find the parasitic drain.
*To determine if you have a parasitic drain:
With the engine and key off, remove the negative battery cable from the battery. Place multimeter leads on both the negative post of the battery and the negative cable end, with the multimeter set to read current. If you're seeing a value over 25-50mA, something is drawing too much current. Pull one fuse from the panel at a time, starting with the small fuses, to isolate the circuit that is causing the drain. From there, you can check each device on that circuit to determine the source of the drain.