A maintenance engineer who knows what they're doing can spot problems in the fuselage via a walk-around inspection, for the most part. Of course, without x-raying the entire structure, you're not going to find everything.
We had a prof in our Engineering Failures class who used to work for a major airline as a maintenance engineer. He said we wouldn't believe how many planes are certified flight-worthy with hairline cracks on the wings. Scary stuff.
