How do I make a Liquid-level-indicator for an overhead sintex water tank
Probably the simplest indicator would be to have a float that raises or lowers a rod of some kind. In this setup a float is attached to a vertical rod that goes through the ceiling of the tank. The rod then angles horizontally to the right to the side of the tank, then down vertically towards the ground. You would then build an indicator with lines painted for every unit of height of water in the tank. As the water level raises the float goes up, lifting the rod assembly, and as water us used the whole assembly drops. 

The other mechanical way I can think of would be to use water pressure to indicate the amount of water in the tank. Assuming this is gravity fed you could install a pressure gauge on the main water feed and then calibrate the pressure levels with the water levels. 

There are other mechanical ways to do this, however if it were me I'd go with an electronic solution of some kind rather than a mechanical one. You could install a low power (even solar powered) electronic system with some LEDs to show your water levels. You could even install a webcam in the tank itself and measure water levels based on lines painted on the inside.