If you want to promote usage of real CPC computers, you can detect CPC emulators by:
*Checking if the memory is initialized with zeroes, as real machines have random memory values at boot
*Configuring the CRTC to make a stop refreshing part of the RAM not to be refreshed. This behaviour has never been emulated. [https://www.cpcwiki.eu/forum/amstrad-cpc-hardware/memory-refresh-plus/ Source]
The way to cause the RAM refresh to fail in both a Plus or normal CPC is simply to stop a few bits of the CRTC address changing (ie. never refresh the selected area).
Generally, only the Row address needs to be cycled, so stopping MA0 through MA7 from changing, and stopping the CPU from reading those rows, will cause data to be lost, quite quickly (generally around 4ms).