There's only one way to test safelights for fog. Take a sheet from bottom of box, lay flat on bench with cardboard covering 3/4 of it, after1 minute move board to expose more of paper, repeat every minute until bored, then process paper. That will give you a step wedge indicating how many minutes you can leave a sheet of paper on the bench under your present system.
This is a very bad, basically useless test.
The problem with this is that paper has a certain threshold exposure that must be exceeded before it records any change at all, and then much smaller additional exposure will cause additional darkening. This is why pre-flashing paper can work to reduce contrast without fogging highlights in the print. A really bad safelight that will badly fog highlights will still pass such a test.
To really test your safelight both Kodak and Ilford give detailed procedures but the gist is this: in TOTAL DARKNESS give small pieces of your paper small but increasing exposures under your enlarger. Process the tests in total darkness. Find the one that results in a very slight graying just darker than paper base white.
Then take the piece of paper for your test and, again in total darkness, give it this threshold exposure. Then lay a small object on it (coin or similar) and turn on your safelight at the distance from the paper and for the time you want to test. Then turn it off and process the paper, again in total darkness. Any discernible outline of the object placed on the paper indicates that your safelight is causing some degree of fogging.
If you've only tested without pre-exposing the paper you may be startled by how much fog even an apparently good safelight can cause with this test.
