How can I determine the start up amps on a 3 phase 5 ton A/C unit?
For a hermetic compressor (what you most likely have in a unit of that size and vintage) -- the answer you seek is right on the compressor's nameplate, marked as "LRA", "Locked-Rotor Amps", under a "Locked Rotor" field, or in a similar fashion, as per NEC 440.4(A):


  The locked-rotor current of each single-phase motor-compressor having a 
  rated-load current of more than 9 amperes at 115 volts, or more than 4.5 
  amperes at 230 volts, and each polyphase motor-compressor shall be marked 
  on the motor-compressor nameplate.


If you, somehow, have a compressor that uses an external motor drive, check the motor nameplate for the information from 430.7(B) point 8:


  (8) Code letter or locked-rotor amperes if an alternating-current motor 
  rated 1/2 hp or more. On polyphase wound-rotor motors, the code letter 
  shall be omitted.


(You'll also need Table 430.7(B) to interpret the code letters if your motor is marked that way.)