There is a telemetry feed for 'mtarg` with three values, the port, vertical, starboard values of the motors in microseconds. 1500 is neutral. mtarg is the target for the motors. So when going forward at half speed, both the port and starboard value should read ~1750ms, but they should be equal.
If not moving forward, but rotating right/left seem work work correctly?
Some mechanical things to check:
If you have the power off and manually try rotating the props, do they both seem to rotate with the same resistance? If one has a bad bearing then it will be harder to turn than the other and may result in a slower rpm for the same motor target speed.
If you visually inspect the props, do they seem in generally the same condition? If you have one prop that has a leading edge that is scratched up or pitted, that hurts the efficiency and I have seen a good 15 to 20 degree turn when attempting to go straight with a damaged prop.