Get a $20 multimeter -- it will make this process a lot simpler
It doesn't crank when boosted? Even when left to charge for a bit?
It could just be your alternator going slowly as mentioned... Or your battery draining slowly...
Try to get it started (roll start, jump, etc) and meter the battery. You'll be able to rule in / out alternator right away if the volts are staying constant or dropping.
When it's even running for a bit and then is shut off, meter again to check if your battery is holding charge.
May be worth investing in an on-car trickle charger to keep your battery nice when it's sitting for periods like that