White side out, unexposed film inside.
Black side out, either exposed film or empty. Either way, not usable for picture taking.
If I'm loading and run out of film, I leave the slide on the empty's partly out, then turn it over (black side out) when the lights go on.
On the rare occasions I accumulate exposed holders waiting for processing I keep them separate from the empty holders.
I never remember which side has the bumps, so I don't rely on those. If there is any reason to doubt whether a holder has film, a quick shake will tell you, make sure to hook a finger over the dark slides though.
However, if someone would come up with an automatic turner-over for insuring the slide is switched after an exposure, he would be doing a great service. IMO at least