You sure you're talking about trochus, and not astrea? In my experience, a trochus' foot can whip around and find just about anything to latch on to, and flip itself back over. Pretty limber guys. (Not saying I've never lost any due to "upside down syndrome". But it doesn't happen often.)
Astreas though... once they're upside down, they're pretty much toast if not found within a few hours. That's why I normally recommend trochus over astreas.
The outside portion of a trochus foot (not the part they "walk" on) is normally black. The same area of an astrea foot is tan, or tan/black mottled color.