Welcome Adam, might be worth putting a post in the noobs section to introduce yourself properly...
....do you have an alarm? if yes look there.
If no alarm...
To test whether you rectifier/regulator is working, check the battery voltage with the ignition off, should be 12.55/12.60ish volts (lower is not good, less than 12v is very bad and higher indicates a an overcharge problem- lead acid batteries 'memorise' their overcharge condition for some time after last being overcharged).
Then run the engine, give it a few seconds at least, then check the voltage across battery should be between 13v and 15v, less than 13v means it probably ain't charging (reg/rect, wiring, generator), more than 15v (14.5v in some cases) will mean the rec/rectifier is overcharging and is goosed. The voltage should also be reasonably stable (providing the idle is stable). If at idle it checks ok, then check the voltages again at 4000rpm (some reg/recs only work at higher rpms, unlikely on a factory custom, but someone may have substituted parts over the years).
Get this sorted and then if you have performance issues move onto that.
HTH
merv