what is the best liquid sensor that can read liquid levels from outside a plastic cup.ie can see thru the plastic to the level
or is the better option to have a reader inside the cup
Point Level Capacitance transmitters, like a Siemens CLS100 or CLS200 can sense liquid level through glass or plastic. Either has a sensitivity adjustment.
I suspect that capactive proximity sensors can do the same at less cost, but they typically have a 'nominal sensing distance". Changing the distance is probably similar to the sensitivity adjustment.
The advantage of sensing outside the vessel is it’s non-contact. The disadvantage is fabbing or customizing mounting for the device.