Rad header tank

If there is no water in the Rad expansion tank where should it come up as a danger

If it has a level sender (easily identified by there being a sensor, and wires, coming out the top of the bottle) (1983 onwards I thnk) then it will flash the coolant light in the dashboard. If you don't have a sender, you have to check it regularly……..

sender is there

Is there any way of telling if it's knackered
should it come on on the dash for a second when turninng ignition on

I'm guessing its a plain old switch, so simply connect a multimeter to it and see if it switches on and off as you fill up the bottle?