Just run a F19 NMR of the TFAA. That should allow you to determine the ratio of anhydride to acid. It will be hard to find TFAA without some acid, even in a new bottle.
I have actually seen the opposite problem, some bottle of TFA acid that we purchased had substantial levels of the anhydride in them, which was reacting with our material. if the TFA is used as solvent for in excess, even a small amount of the anhydride can provide lots of it for reaction with certain compounds.