My spacecraft knows it wants to add a certain delta-v in a certain direction to its motion, relative to the stars. It will calculate current mass based on propellant usage history, which hasn't been much so far, and it knows the engine's well-characterized thrust, so taken together, it knows that it needs to burn for a time T in order to produce the correct magnitude of delta-v.
Its internal contents tend to shift, so that the precise location of the spacecraft's center of mass is unpredictable. Luckily the engine is gimbaled, and it can adjust the gimbals continuously during the maneuver to make sure it keeps the center of mass directly on the thrust axis. It does this by detecting rotation with the star cameras and gyros and gimbaling to null the rotation.
How does my spacecraft know if the thrust is actually pointing in the right direction? The stars are very far away and there are no handy planets or asteroids nearby, so while it knows attitude, how can it determine that the direction of the delta-v vector is correct?
I suppose internal accelerometers (inertial guidance) that have been cross-calibrated with the star cameras would be one solution, and exchange of radio communication and doppler information with Earth would be another.
Is that it? Inertial sensors and doppler, or is there any other currently used deep-space spacecraft technology that can measure the direction of delta-v in real time?