Take the paper unexposed, cut it in to 3 strips and develop for 1 min, 2 min and 3 min (I am sssuming FB paper here) in fresh developer. After fixing and drying, compare them...are they the same colour or is there a graying with development time? Fogging shows up as greying (effectively exposure) of the paper. The development time changes helps since if the paper is old, it may be slower and need longer times to show the fogging (wierd I know, but speed and fogging kind of work against each other I have been told)...
To test the developer. Take a strip of unexposed paper. Expose for two seconds with the aperture at full open on the enlarger (no neg!). Then develop for the recommended time. Then fix and dry. Do you have a true black? If not, then the developer is likely depleted...
Rgds, Kal