Measute spectrophotometrically the activity of the enzyme at 340 nm in whole cell extracts, with glucose and NAD or NADP. The reduction of NADP/NAD will increase the abs.
You can follow product formation by HPLC, possibly with radiolabelled glucose but if you use glucose as a substrate, and just measure A 340nm, the reaction may be semi quantitative because the glucose can be broken down by other enzymes and then the reaction intermediates could be then used by other dehydrogenases.