Get a battery tester .. the little ones that measure specific gravity of a sample of the battery acid. Use a volt meter to check total voltage. For each cell you should see from about 1.8 volts, up to about 2.3 volts.. even on a discharged battery.
If it's far less, one or more of the cells is probably shorted or dead.
Top them off with water and attempt to recharge the ones that are discharged. If one doesn't accept a full charge, toss it out.
Batteries that sit in a discharged state for more than a month or so start to sulfate. This sulfate coating on the plates cannot be removed and it reduces the amount of charge the battery can accept and deliver.
Consider replacing the wet lead-acid batteries with gel cells.. if you can fine one that fits the battery box. Gel cells have a gelatine-like stuff instead of watery liquid. The distructive sulfation process proceeds much slower and they hold a charge longer when they are left unattended.