Hi,
since you didn't give any further info,I'm not sure about your current knowledge regarding MIDI and Reaper.
Just in case, for the basics,I'd suggest taking a look here
http://wiki.cockos.com/wiki/index.php/Recording_MIDI
If this is all well known already, for your actual task, you could insert ReacontrolMIDI (a plugin included with Reaper) on a track and enable the "Log" function, record arm the track and than you can see if / what the controllers are sending when you move one.