How would a test pilot determine the MTOW of an aircraft. Does the manufacturer predict this value?
Pilots do not determine the official MTOW of an aircraft. MTOW will be estimated by the manufacturer during the design of the aircraft based on the required payload of the aircraft and the design configuration. It will be adjusted based on the detail design of the aircraft to comply with regulation requirements.
§25.1533 Additional operating limitations.
(a) Additional operating limitations must be established as follows:
(1) The maximum takeoff weights must be established as the weights at which compliance is shown with the applicable provisions of this part (including the takeoff climb provisions of §25.121(a) through (c), for altitudes and ambient temperatures).
MTOW affects many other aspects of an aircraft's performance. It will affect stopping distance after a rejected takeoff, takeoff performance (including engine failure), climb performance, taxi weights, and many structural aspects. MTOW will be chosen so that the requirements for all of these conditions are met. This is why new engines or winglets which improve performance will allow a higher MTOW.
Of course, as always, the pilot has the authority to set their own personal MTOW more conservatively, if they feel it is necessary for safer operation of the aircraft.
MTOW is defined and then verified by the manufacturer.
During "verification", tests and other analyses are conducted to show compliance to all MTOW related requirements. For example, landing gear analyses and tests are conducted for their corresponding FAR23 or FAR25 regulations. Flight Loads analyses/tests are also conducted, so that the structure can withstand all foreseeable loads during the entire operational life of the aircraft.
During the development of the aircraft, the MTOW can be updated (which is painful and costly) and during flight tests, final tuning can be required. Inceasing MTOW is more costly, in terms of development efforts and re-engineering of the aircraft. Decreasing MTOW would probably limit the operational capacity of the aircraft(less fuel, or less payload).