Even without checking the engine instruments, the most obvious and immediate engine failure or loss of thrust "indication" prior to V1 would be an adverse yaw towards the side of the failed engine; Unless in the case of a trijet where if the center engine fails, without asymetrical yaw, the pilots would be alerted by at least one or two bright "Eng-2 Fail" warning lights on the glareshield panel. The "Eng 2 Fail" warning light(s) would normally illuminate if N2 has decreased below something like 80% after having been advanced above something like 83% during the takeoff roll (in ground logic with flaps set, etc).