Visual check of the connection:
Check that the plug is fully inserted into the earth contact and that the connecting cable is properly attached to the sheet.

Measuring with an earth tester or multimeter:
A simple earth tester for grounding products can show whether a connection to earth exists.
Alternatively you can use a multimeter to measure the resistance between the sheet and the earth line. A low resistance indicates a working connection.

Below is a video description that illustrates the procedure:
https://www.youtube.com/watch?v=EISRiHRq8nA

If the voltage drops for you as well, everything is connected correctly. If not, try another earthed socket: if you notice a difference, have an electrician check the earthing of the faulty socket.