How do you know which outlet is controlled by a switch?

Plug a lamp into a working socket of an outlet to ensure the lamp works. Then turn off the switch and plug lamp into all outlet sockets, top and bottom outlets until the lamp doesn't light. Then turn on the switch and the lamp should light. Often a switch in a bedroom is only connected to one socket in a duplex outlet. It is possible to remove a jumper in an outlet to isolate the sockets for just this purpose. Often an electrician will install the outlet upside down (The third prong pointing up, to identify the outlet.