My number one symptom of rotting design is a lot simpler, I keep postponing touching the code... If I do this for more than a couple of days it must be really rotten.
I believe this was mentioned in the Pragmatic Programmer. They used the analogy of broken windows.
nobody likes to work with legacy code... managing to work with legacy code and without a steady stream of complaints is what separates a good programmer from a noob.
Actually that is what separates the professional from the hobbyist (I've seen both bad professional programmers aka 9-5ers and amazing hobbyists), and when I work professionally both in my startup and in a previous life as an employee I put on my waders and jump right into the thick smelly bog. But especially when you take a couple of years of vacation and work for fun on projects that interest you, you find out that sense of ickiness is probably the best indicator of code rot.
One skill that I'm growing to appreciate as I become more experienced is the ability to find a problem and determine if the best action is to do a minimally invasive change, a workaround, or a full restructuring. I don't always make the right call but I'm getting better at it.
Oh man, by your definition I have some seriously stinking code...
Not necessarily, the question is do you procrastinate on parts of it significantly more than the rest, if you do that part probably does stink.