X-Plane Dataref-Editor

To check values of specific datarefs you can download and use the "Dataref editor" from X-Plane.
Source: http://www.xsquawkbox.net/xpsdk/mediawiki/DataRefEditor

This plugin will display all datarefs for the airplane you are using. In my case I wanted to check the values for the Fuel-Flow. The output is that it varies between 0.003 and 0.009 kg/sec.

Screenshot of Fuel-flow in the C172:

 idle

full throttle