For the electric water storage heater there are two safety devices.

One is an electrical RCD(residual current device) & the other a PRV (temperature/pressure relief valve)

There is usually a test button to trip the RCD to see if it is working so press the trip button once a while to make sure the RCD is working

For the PRV you will need to see the hot water storage PRV drain pipe is not blocked and you can sometimes test the valve by manually pressing but hot water or steam will shoots out so be careful.

When the PRV fails the hot water cylinder becomes a cannon and will explode or rocket out its mountings making a big hole in the ceiling or take down a wall. You don't want to be nearby when that happens.