The whole idea behind prepping is being prepared.
You should develop your plans around the most likely disasters (natural or man-made) you will confront in your area.
You should gather your equipment, gear, bags/packs, transportation, and supplies the same way.
Then, if you have planned logically, and prepared according to what you might have to contend with, knowing when the SHTF is a matter of being situationally aware. But by then, your BOB/BIB; BOV; INCH; BOL; and related preps should be able to sustain you and allow you to survive whatever threats you face.
In other words, if you are truly prepped, when it comes, you'll be ready.
Knowing when is really very easy in a society like ours in America where there is a 24/7 news and weather cycle, where you can afford an emergency warning/weather radio; where you can have the Internet and GPS in the palm of your hand; and where you train yourself to be alert to threats and watch for warning signs.
If you pay attention to what is happening around you, you will know BEFORE the S is going to HTF.
When I hunt I listen to the birds singing in the trees. If that stops suddenly, you better look to see why....
One example of many....