How do you know how close a lighting strike is?
When you see the flash, start counting, when you hear the boom, stop, one mile per second, if it takes 5 seconds from flash to boom, it's 5 miles away. (Or so they say, not sure how accurate that is).
When your hair is on fire!!!!
When you wake up in the ER without your tachymeter or remembering much of what happened. You will later know in retrospect it was close, really close.
whelash of lightning happens, just start counting and when you hear the thunder stop counting then whatever number you stopped at is how many miles it is away