If you bought it from a private owner, I would think that they would show you or at the very least if the slide was out somehow the slide had to be put in before you towed it. So if you cannot find the switch, ask the folks that you purchased it from. If it came from a dealer, by all means someone there had to have operated the slide too. My neighbor has an older camper and the his switch in located on the side of his kitchen cabinet on his layout, right out in the open.
If your unit has a crank, this explains how you can do it if your electrical mechanism is not working . By the way what model and type do you have? Maybe look on you tube too. There is not much that's not on it. I say ask you tube
https://traveltips.usatoday.com/manually-open-electric-slide-travel-trailer-103311.html
And there are several articles on this link with similar type units.
https://www.justanswer.com/rv-motorhome/bcpgs-99-prowler-fifth-wheel-slide-nor.html