It is very easy to hear each of the three drivers. No need to touch them. Just move your ear up and down. It will be quite obvious if one isn't working.
It is quite likely that the problem lies in something else. It could be as simple as a poor connection or soldering joint in one of the cables.
It is quite likely that the problem lies in something else. It could be as simple as a poor connection or soldering joint in one of the cables.