I assume that you are referring to the ANSI 150 pound, 300 pound, 600 pound,etc. flange ratings which provide flange dimensions and allowable operating/design pressure/temperatures.
You will have to find the the ANSI Class and MOC (material of construction) stamped on the perriphery of the flange and then go to a table of pressure temperature ratings for the allowable values. Do not assume anything.
If there is no rating (markings) as stated above you are in somewhat of quandry as you really can't do a legitimate calculation without the MOC. Even if you assume the lowest values of acceptable flange materials and do the required calculations they still may not be acceptable under most jurisdicitional requirements. You will have to do a PMI (Postive Material Identification) on each flange.
Before you do all this work consult with your jurisdictional authority.