In theory it's about 1.1*Vnominal, so count how many cells are in series, and multiply 1.1*n*1.2V. So, 1.32V/cell, maybe a little higher. 1.5V/cell seems too much.
18V/1.2V = 15 series cells, but double check if they really are 15 cells inside, sometimes cheap drills have lying labels. I have found a battery pack from a toy RC car with a metal rod instead of one of the cells.
18V*1.1 = 19.1V
15*1.5V = 22.5V, might be too much as a stop voltage, though NiCd batteries can take more abuse than other chemistry types.
If your power supply indicates the voltage, a good time to stop is when the total voltage start to decrease (under a constant charging current). At first the voltage goes up, then after the batteries are fully charged, the voltage decreases a little. For 15 series cell you might see the total voltage slowly decreasing about 0.5V, maybe more when overcharged, but you don't have to wait that long.
The idea is to disconnect the charger when the voltage is starting to decrease.