Gainer,the clip on meter I have was made by Leica (actually for) that couples to the shutter dial. I could probably use that to check the lamp as you suggested. Although since my exposures within a session are consistent I doubt that is the problem. Sometimes to check an exposure I'll leave the darkroom with a wet print and put it in the microwave to simulate dry down. So from the time I exposed the print/developed/fixed/dried/maybe had a snack could be 20-30 minutes. When I make an exposure change whether I'm satisfied with the change or not it usually is consistent. But it is still good to know.
I will however use distilled water to mix the Dektol and then again when I dilute it 1 +2 for the session. I am going to throw out the Dektol I mixed last week and start again since it is cheap enough. I will also certainly be more consistent with the temperature of the developer.