How can the pilot tell the center engine failed on a piston aircraft with three engines? There are a few aircraft, such as the Britten-Norman Trislander, that use three piston engines -- one on each wing and one mounted along the centerline somewhere.

This is fine and good, but how do you tell in a timely fashion that the #2 (centerline) engine has quit?  The engine gauges are in no hurry to tell you, as per Randy Sohn's Warbird Notes #6, and there's no adverse yaw because it's a centerline-mounted engine, but you still need to know and quick because unless you can get that prop feathered, there's a giant drag bucket hanging off your vertical tail!
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.