[interminable]

I obtained the IR spectrum of Mesitylenetricarbonyl molybdenum(0), and there are two CO stretching vibrations at 1960 and 1875 cm^-1.

I know that the two IR active CO modes are the A1 and the E from the irreducible representation. I found a paper that tells me that the one at the higher wavenumber corresponds to the A1 mode. How do you know this is the case?