it should say on the engine case right by the filler...if not, you should have a sight glass/window on the clutch side of the motor, and the oil should be somewhere in the middle of the glass.
If you have no window/sight glass...it's usually just shy of 1 quart.
The manual I have for an 88 KX125 calls for 0.65 L.
If all else fails....a simple call to your local Kawie dealer will get you the answer you need too.