method and assembly for inspecting the contents of a connection box or a utility system such as a sewer , telephone electrical , gas etc . via an inspection plug . the inspection plug provides an inspection port and is insertable into a hole in a panel and is attachable to the panel by fixing elements such as a nut assembly , a locking mechanism , prongs , etc . the inspection plug may include a body , a peripheral shoulder , and a resiliently braced cap pivotally mounted in the body to close the inspection port . the peripheral shoulder supports the plug in place in the panel . the cap normally closes the inspection port , but is yieldably openable to allow an inspection probe or other device to gain access to the internal atmosphere for testing within the utility system or electrical connection box . the interior space of the body may contain a gel material , and the cap can be substituted with a resilient closure member having self - sealing openings therein for receiving an inspection probe . the plug may be adjacent to a gas detection alarm system .