Hard starting 91 Jimmy sle

How can I test the alternator?
my jimmy has been not wanting to start without a jump.I'm thinking the alternator may be dying and not charging the system.
reset the brake light switch at the pedal when it got stuck on and drained the battery overnight.
how do I check for a short somewhere, if it's not the alt?
something is draining the batt. Or it's just an old batt?
please any help is greatly appreciated.