When you own more than 8 thermometers
When your circadian rythym wakes you up at lights on time, no alarm needed
When you always have that sneaking suspicion you left the door open, and the cats got in there
When you walk out of the grocery store with planting pots, perlite, soil test kits, distilled water, and nothing else. In mid winter
When you do your own apartment repairs, even over $20, just to avoid calling maintenance
When you go to get your mail in the middle of the night, just so you can see if there are any light leaks
When you have declined sex to attend to urgent horticural matters <-----(has not happened to me yet, but I imagine this is when I will know for sure)