Well, the basic diagnostic criteria for delineating between "just being weird" and "having a mental illness" is that it's having a significant detrimental effect on your life.

For example, you spend so much on prepping that you can't afford other necessities. You have so much junk that you have hygiene problems or no room to move. You are so obsessed with it that you forgo social interaction, lose friends, lose your job. Etc, etc.

But of course, that's sort of the end state. Some of the signs that you're getting there... I think basically just a never-ending anxiety and obsession, seeing signs of the apocalypse everywhere, etc... Ideally, prepping should be kinda fire-and-forget: you do some stuff so that you can have more confidence as you go through your everyday life, knowing that you don't have to obsess about the what-ifs and spend your waking hours on zerohedge.com.