Should really charge it then test it with a proper battery load tester. All you can do on your own is get a multimeter and check the voltage. Without it running and having sat for a 1/2 hour or so the voltage should be approx 12-12.5 volts. Start the engine and if the alternator is good the voltage at the battery terminals should be 13-14 volts. If without the engine running the battery voltage is 12 or less this may indicate that the battery is not holding a charge. Even then the voltage should go to between 13-14 volts when engine is started showing that the alternator is working to charge the battery. If when started the battery voltage stays around 12-12.5 volts or less I would suspect the alternator. Just to double check there is the main power out terminal on the back of the alternator. With the engine running check the voltage. If 13-14 volts is present at the alternator but not the battery the circuit in between the alternator and battery is suspect. If the voltage at the alternator is the same as at the battery (12-12.5 or less) the alternator or voltage regulator may be defective.