How do CFIs typically determine proficiency for complex and high-performance endorsement?
This Advisory Circular describes what should be examined when transitioning (and ultimately endorsing) a pilot for complex operations. 


  This publication is offered as a guide to the procedures and standards
  to be followed for a thorough and comprehensive checkout in these
  airplanes. The conscientious application and adherence to the scope of
  coverage recommended in the syllabus should result in a more
  competent, effective, and efficient pilot.


The document is fairly short and I suggest you look through it, for brevity I wont copy it here as its pretty much all relevant. Its important to note that this is simply what the FAA suggests (albeit strongly) and its not required all the described things be covered in a checkout. Although the instructor may be assuming some liability by signing you off without a proper checkout.