The proper dilution of the appropriate cleaning solution can be applied to the test panel to see what happens. If the wrong products or dilutions have been used, they'll bleach the mortar or etch the joints or both. Some minerals in brick may react with cleaning solutions and the result will show up as stains on the units or on the mortar. It's also a good idea to build that test panel as soon as the job started, so that cleaning solutions can be applied to it well before the actual job is finished. If the test panel cleans up properly with the methods and chemicals being used, chances are good the walls will clean up properly, too.
Timing is critical.
Manufacturers of cleaning solutions recommend waiting seven to 28 days before applying their products. But leaving scaffolding in place for extra weeks or a month is expensive and may not even be possible. Many contractors are forced to start the cleaning process within two days of completing the actual construction. Cleaning up so close to completion means that even more stringent precautions need to be taken, because there is a strong possibility that the mortar joints might be etched or that color might be removed from the joints. Be sure to use plenty of water.