a voltmeter is the next step. (you can usually purchase a decent one for $20 or so, or borrow one)
when the car is running the alternator should put out in excess of 12 volts (usually around 14)
I would charge the battery to full capacity off of the car, then put it in and measure the voltage when you start it. (the battery should not drop below 9v or so while cranking and then return to 12+)
and clean terminals are necessary.