Back flow device inspections ? ...
You didn't state what type it is. There are backflow preventers for drain lines and afterwards for water systems such as sprinkler lines.
A crapper flapper?
Usually there is a lid that can be unscrewed. Secure the home (so not a soul flushes or otherwise uses water while you hold this open). It will likely have need of to be cleaned. Use gloves. If this port is inside, a fan for fresh nouns might help!
Answers: A backflow preventer is a little device that screw onto your hose bibbs