The rocket doesn't know, but an outside observer will. If you're on the rocket, you experience a constant 1g force. The rocket doesn't "feel" that it's harder t

The rocket doesn't know, but an outside observer will. If you're on the rocket, you experience a constant 1g force. The rocket doesn't "feel" that it's harder to accelerate, but an observer that remains at rest relative to the rocket's starting point will see it speed up less and less the faster it's going as it approaches the speed of light.