Visual testing is simply stated as looking at a component with your eyes. VT testing can be performed with flashlights, optical prisms, microscopes, comparators, borescopes & video probes.
In 1997, the FAA (Federal Aviation Administration) released an advisory circular 43-204 Visual Inspection for Aircraft. The 242 page AC is an acceptable training guide for an aviation visual course and it should be reviewed by anyone performing aviation visual inspections for the detection of flaws and corrosion.
A Aviation A&P inspectors should be specifically trained in visual inspection with an emphasis on proper lighting and specific equipment utilized at their facility. This training should include borescope, video probes, optical prisms & magnification apparatuses. Training should be documented and on file in personnel folders with current valid eye examinations. If personnel perform NDT, they should also be certified to and NDT personnel certification procedure for visual testing.
Visual testing of welds is the primary use of NDT inspectors performing visual inspections. Visual Testing (VT), Visual Examination (VE), and Visual Inspection (VI) are all commonly used terms, but testing and examination or inspection are clearly defined as separate practices in some codes and standards.
Visual testing of welds and welder certification is a specialized industry of NDT that must be addressed separately. The primary thing you should know is if your code requires a CWI (certified weld inspector) or not.