The neutral beam injectors for ITER are designed to deliver hydrogen or deuterium atoms accelerated at 1 MV for a total power of 34 MW. Design of the neutral beam and of the ion source test facilities is presently ongoing. Among the various activities, also the definition and assessment of necessary diagnostics are being carried out. Within this framework, requirements and constraints of the diagnostic system will be discussed in the present paper with the aim of providing data relevant for protection, operation, characterization, and improvement of the performances of the test facilities. The definition of the most suitable locations for the diagnostics and of their preliminary integration into the mechanical design will be proposed. Proposals for the extraction of signals from the high voltage components will also be presented.