the 'wall wart' sort of power supply is usually poorly regulated in voltage; it may actually be as much as 18V OC, so when you test it, put it under a load. (Not a short circuit. . .)
probably the BEST way to test it would be to hook a Digital Ammeter into the circuit, and actually connect it to the battery. if you get 100+ mA into the battery (depending on state of charge), then it'll be fine for your needs. if you do go that route, be sure and check the volts across the battery during the charge regularly for the first couple of charge cycles. . . you don't want the volts to drift up into the 'fried battery' range (above 14.5V) and stay there.
-Dan
PS: most hobby shops have cheap battery chargers for that size of battery. . .