Use the load feature of the Duspol to apply a load between L and PE.
This will not trigger an RCD with a rated residual current higher than 30 mA, though you shouldn't have an RCD rated over 30 mA installed in your home, as that's the maximum permissible rated residual current for personal protection anyways.
If your Duspol does not have the load feature or if you do not have a Duspol, you can still test the GFCI by shorting the neutral to ground. However, to do this, it is important to ensure that a device is currently drawing power from the circuit being tested. This is typically the case in most household settings.
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