asuryan:
Does a simple LDR and LED sensor should do the trick even when the fluid is pretty clear? Or are there any other ideas?
Using light sounds like a promising approach but It depends on the fluid really.
If the fluid is very clear, you might be able to detect refraction, or a reflection from the underside of the meniscus if you get the LED and sensor arranged suitably. Also if the meniscus is reflecting, that would cast a shadow somewhere that you could probably detect.
You need to do some experiments!