Pro tips for inspecting structure during condition inspections?

Does anybody have tips on how best to inspect structure during a condition inspection? I get the relatively easy part about looking for cracks, ripples, smoking rivets and other visible things on skin and easily accessible structure, but how about spotting hard-to-see problems in wing interiors away from inspection plates, tailcone interiors, etc?

All ideas are appreciated, and I'm especially interested in how the IA/professional maintenance folks do this.

Many thanks -

Dave

Reach into the hard to see places with your cell phone and take a picture. Then you can blow it up and take a close look.