Visual inspection of the connection:
Check that the plug is fully inserted into the earth socket and that the connecting cable is correctly attached to the sheet.

Measurement using an earth tester or multimeter:
A simple earth tester for earthing products can indicate whether there is a connection to earth.
Alternatively, a multimeter can be used to measure the resistance between the sheet and the earth lead. A low resistance indicates a working connection.

Here is a video explaining this in more detail. 
https://www.youtube.com/watch?v=EISRiHRq8nA

If the voltage drops for you as well, everything is connected correctly. If not, check at another earthed socket; if you notice a difference there, call an electrician to have the earthing at the faulty socket checked.