The "cleared level" is interrogated by the Mode-S radar sensor from the airplane. It is usually what you have turned in on the MCP. It is a part of the mandatory enhanced surveillance capabilities of the Mode-S transponder.
http://www.eurocontrol.int/msa/publi...craftList.html
Also worth to know is that IAS is interrogated, too, and any deviations from a mandated speed are now subject to comments from ATC.