How can the pilot tell the center engine failed on a piston aircraft with three engines?
If the engine quits you will lose some thrust which will affect the plane's handling. You may even begin to descend slowly which the pilot will notice. 

Most importantly you have engine instruments which will tell you, quite quickly, the engine has quit. If you have one, your manifold pressure will drop since you are no longer producing power, or your tach will reflect what is happening.

If all that fails you I would think there would be an audible difference if the engine cut out. 

This is the cockpit of the aircraft in question:


(Image Source: Jetphotos.net)


It does show independent MAP and RPM gauges for each engine as opposed to some planes that have a single gauge with multiple hands (one for each engine). You would surely be able to see an altered reading on one of these gauges.