There is no requirement to be licensed to do truck inspections, but you must be certified. Requirements can be found in 49 CFR Part 396.19. Basically, you must complete a training program, have experience as a mechanic, or a federal or state inspector or similar experience. The inspector must also be familiar with Parts 393, 396, and Appendix G of the Federal Regulations. Forms for conducting the inspections can be obtained through motor carrier associations, truck stops, etc.