How accurate do you need to be?
Quick and dirty method: Use the agitator motor HP as the heat input to the batch. This is a high-spot estimate unless you are overamping the motor.
More involved method: Google “mixing power”, or look in Perrys. You’ll need to calculate Re for the impeller, determine if impeller flow is laminar or turbulent, select the correct equation, and hopefully have a standard blade design so the empirical agitator constant is readily available.
Most involved method: Programs like Visimix, which I have used (no affiliation), can give mixing power after you enter the geometry of the vessel/agitator and liquid properties. It also many other mixing evaluations. However, you need to purchase a license for it.