testing VDO water level sender

[enrigenna]

On a lagoon 500 one fresh water gauge (VDO) for two different tanks with switch to select.
They both started reading full even for empty tank, I need to figure out if it is the gauge that needs to be replaced, or the senders.
The senders are the capacitive type, how can I test them?

[smac999]

to test the gauge. unhook sender. switch between connecting ground and then +12 to sender post. it should move empty and full.