They use the hand to "SMELL" the burning oil. Its easy to detect when its difficult to see the smoke coming from exhuast.
Another indication would be to check for "Wet Carbon" in exhuast. (meant for mohsinwahhab). Sweep your finger inside the Exhaust hole if your bike, and inspect the carbon under your Headlight. If you find the "shining" particles in it then it will indicate Wet Carbon, which would be the result of Engine Oil burning.
In either case, changing Piston Rings is necessary. Otherwise, it won't be long before the bike would start asking for another head opening.
Its also advised to get the whole head re-done, which would include Piston, Piston Rings, Valves, Valve Seals etc ... otherwise your Piston Rings won't last more than 6-8 months before asking for replacement again. (common problem in CG125)