borescope

Short definition from Wiktionary

Noun

1. 1
   An optical instrument used for seeing inside tight spaces, consisting of a rigid or flexible tube with an optical relay inside so that the view through a lens or window at one end of the tube may be seen from a lens or eyepiece in the opposite end of the tube.

Verb

1. 1
   To see inside or examine (something) using a borescope.