I wouldn't bother attempting to determine the existing rating, if for no other reason that it may not have been properly sized to begin with, and / or recommendations may have changed since then. Previous appliances may have been replaced or updated, or additional appliances may have been added. If it's a commercial kitchen there are likely to be applicable codes that may also have been changed since the original installation.
Better to start with the manufacturer(s) of the exhaust fan you are contemplating installing, and use their calculations. Most of them have online calculators that take into account the kitchen's volume, cooking appliance BTU, ducting considerations... etc.