If you ask 10 photographers how to do a certain thing you will probably get 10 different answers. Just to confuse matters they may all be right. So, here are two ways of checking for safelight fog. The first is one variation of a popular and wrong way to check.

The wrong way.

With your safelight off place a sheet of unexposed paper on your baseboard then place a small opaque object, a coin is usually favourite, on the paper then turn on the safelighting for about seven minutes, a reasonable time to allow you to expose and process a normal print.
Turn of your safelighting and process the film as normal.
If your print shows the shape of the object( not exposed to the safelight) lighter than the rest of the print then you have fogging which, if your darkroom is free of light leaks, comes from your safelight. Fair enough.
Where this test falls down is in that if your print is an even, pristine base white it does not mean you don't have safelight fogging.
And why is that ? I here you ask. Now I am going to have to try and explain.
Print and film emulsions have a threshold value below which an image will not develop. Say for instance a sheet of printing paper needs an exposure of 5 seconds before it will show a tone over base white then the threshold is about 4 seconds. That means if you expose the print for five seconds you will see a grey tone but if you expose for 4 seconds you will get base white.
Now back to out test.
Although your test print was white you have no way of knowing what exposure may have been received below and up to the threshold value . As exposure is cumulative (it adds up) when you expose your prints any additional exposure from your safelight is going to sneak in and fog your highlights, only you have checked and think you have no safelight fog and will spend hours trying to get your print right.

Next a better safelight test.