Noon question, but: How do I safety check that there is no light in Darkroom / Darkbag?
If you are talking about locating a light leak in a changing bag for loading film into magazines and cameras without a darkroom available…
Periodically, to check the condition of your changing bag, put a bright light inside the bag through one of the sleeves. Close up the bag. Sit in the darkroom or a closet with no light for a few minutes to let your eyes acclimate. Turn the bag over. Manipulate it. Turn it every which way. Look at it from all angles and pretty soon, you'll see every pinhole if there is one.
It's easier to see a bright light in a dark room because your eyes will dilate within a few minutes to rival a sensitive emulsion.
In the darkroom, place a piece of the material you will be developing, flat, face up, on the work space. Place a handful of coins on this material. Using a timer, remove a coin every 5 minutes. 12 coins will cover 1 hour. At the end of the time span, develop the film or photo paper. A perfect darkroom will leave no evidence. An unsafe darkroom, reveals circular images of the coins. This test will tell you how long it safe to have work out.