A simple test on presence of micro organisms can be done before and after disinfection. Axcentive suggest the use of dipslides for this. The main advantage is that you measure exclusively microbiological activity. By selecting the right dips, you can measure even specific bacteria (e.g. Salmonella, E-coli). The disadvantages of this system are that it takes 24 hours for the result and that you need an incubator. The latter is not a big investment but makes that you do not have a totally handheld system. Other systems, but less suitable are based on measuring ATP (general hygiene) or DNA (expensive).
For more information on available dip slides in the market please follow for example this link: www.dimanco.com.