There are a number of ways to determine the IDR, but the simplest and most readily available is likely using a Woods apparatus and the Levich equation. This amounts to a rotating disk attachment that can be used with most USP setups. Also, the Levich equation (while somewhat involved to derive) is pretty straightforward to apply.
This is an older method that is probably used moe than any other, so you should find a lot of material on this method in the literature and on the web.