In passive hydroponics, a gauge to check the level of water – or rather the nutrient solution – is essential.
Though it is very easy to build one yourself, you might who prefer a “real” gauge. There are quite inexpensive.
Here are some links to webshops where you can find water gauges for hydroponics.
Lechuza, one of the manufacturers, sells them directly online:
Some grow shops in Holland sell them cheaper:
And in the USA: