The reaction of aldehyde dehydrogenase with an aldehyde results in reduction of NAD+ to NADH. This can be monitored as an increase in absorbance at 340 nm using a spectrophotomer or absorbance plate reader, or an increase in fluorescence (EX 340 nm, EM 460 nm) using a spectrofluorometer or fluorescence plate reader. You supply the aldehyde, NAD+, and ALDH in a suitable buffer. Check the literature on your enzyme to identify a suitable buffer and to find the substrate Kms.