you can read out the highest TAS in your performance tables. on flat rated engines is mostly occours in the region of the highest altitude where the engine is able to maintain max torque and /or IAS redline is not a limitong factor. - usually in the mid tenths for turboprops.
flights are conducted usually higher than that because fuel flow in high altitudes drops more significantly than the TAS does so the overall efficency is higher.
the same applies for tubocharged pistons.
cheers