Visual testing is probably the most important of all non-destructive tests. It can often provide valuable information to the well-trained eye. Visual features may be related to workmanship, structural serviceability, and material deterioration and it is particularly important that the engineer is able to differentiate between the various signs of distress which may be encountered. These include for instance, cracks, pop-outs, spalling, disintegration, colour change, weathering, staining, surface blemishes and lack of uniformity.

For crack appearance on concrete structures, aside from the common tools such as measuring tapes or markers to facilitate the inspection, crack width ruler is useful to determine the crack widths for structural or non-structural crack classification. In the case where monitoring of crack widths is required, crack width microscope or crack width gauge can be used to assess any possible movements that may result in the increase of the crack widths (live crack).