The fast slow indicator in the attitude indicator is connected to the INS. It actually does what the A/T would have done, so even before your speed deviates, the doughnut will deviate. Keep the doughnut in the middle, and you will be just fine. The doughnut reacts so quickly that you just move the thrust levers until it is centered.
If you are level you can also use the mach indicator, as it has three digits. The third digit will tell you if you are increasing or decreasing speed.