Change in airspeed, noise change (the sound of one negine winding down and the other increasing to take the load), change in a/c attitude due to reduction in thrust, depnding upon what causes the failure, change in indicated pressures, fuel flows, warning indications illuminating, engine instrumentation, modern a/c will show theres a problem in the health monitoring equipmentwhich will be alerted on the MFD's. Sometimes a pilot will just get the feeling through "seat of his pants" that something is amiss.