Without any markings, the only way is to fully charge, then do a test discharge with a known load (say a resistor calculated to result in 500mA at the nominal voltage), logging the state at intervals, until its discharged. Since it must be old (if it came with the discarded charger) then assume a low capacity (say, 700mAH) and charge accordingly until it measures as if it was fully charged. Say 50mA for 24 hours. The low charging rate should ensure that no damage occurs due to overcharging.
Once you know how long you get at half an amp, divide the time by two and you have the AmpHour figure. Multiply by 1000 and you have mAH.
