About 99% of the time, homeowners can't do their own inspections. They can find leaks, they can see missing shingles, but they can't check for all of the problems during an inspection.
In general, you can actually let your roof go for a few years if you haven't had any problems. You can get by with inspections done by yourself for the first five years of the roof's life. From then on, it's best to hire a professional.