Usually the top plug is the one the switch will supply or deny power to. Rarely I suppose they might switch both plugs in the same box, but I've never seen that. When I had my house built, I specified which outlets I did NOT want to be switched. It might be a code requirement for rooms that do not have ceiling lights. But I know that it's NOT required if there is a ceiling light.
There's a more "elegant" solution. Your hardware store sells a plastic cover. You place it over the switch you do not want anyone to accidentally flip. The cover is "hinged" (it's made of plastic) so you can open it and flip the switch, should you want to, without having to unscrew the plate again and remove the cover. It snaps shut.
Now, here's another scenario a family member is dealing with:
Newly purchased condo. No ceiling lights in the Living Room or either bedroom. Not a single plug is switched; they've all been tested. One switch you would suppose to be tied to an outlet has been replaced. No joy. The original switch must not be defective. Next step? ...maybe replace an outlet?