This is a terrific, thought provoking question that you ask in this topic WPW. I'm surprised that more people haven't jumped into this one.
For the record, I carry a weapon on me at all times, so that to me isn't an indicator that the SHTF.
SHTF can come in many levels as mentioned. It can be individual, regional, national, or planetary. It can be caused by a natural or man made disaster, which would also include inappropriate actions by government. It can also as you mentioned (an excellent example), come in the form of the frog in the pot syndrome. Sneaking up on us slowly, giving the effect of new normal with every downward change. To me it is when your life is turned upside down to the point that society doesn't function for you as it did.
To me indicators that the shit has hit the fan are:
*Mass casualties from anything, be it disease, natural disaster or bullets flying.
*Mass architectural destruction form whatever reason.
*Riots for whatever reason.
*Shortage of food in stores for whatever reason.
*Money loses it's value.
*The military is called out to restore order.
*Anytime any level of government says that a curfew is mandatory.
*Registration or confiscation of firearms.
*When police are not able to enforce civil laws or have gone home in mass numbers.
Those are just my indicators from off the top of my head. I'm sure that I'll think of others with more thought on the subject, which I plan to give it. Thanks for bringing this topic up.