I think you should go for western blotting and then proceed for enzymatic activity assay of taq pol to confirm.

Enzymatic assay would help, but still needs standardization. Check if there is an antibody available for western analysis. If not, Mass spec analysis will give a proper identification of your purified protein.

I agree with Ramachandramouli Budida. Intact protein mass spectrometry will give you the mass of the protein, which you can compare with prediction. DNA polymerase activity assay will confirm that it has the expected activity. If the mass is not quite as expected, as sometimes happens due to post-translational modification of the His-tag, you can use trypsin digestion and mass spectrometry to get some amino acid sequencing done to confirm the identity of the protein.

Thank you Ramachandramouli Budida and Adam B Shapiro for enlightening me more in this context.