You not only need to find a support structure in the ceiling, you need electrical wiring as well. If you have an electric box in the ceiling you are half-way home already. In condos, they are most often surface mounted on a rafter anyway. To find studs or wood members below drywall, buy a stud-finder. You can find it in the home improvement store. The finder can detect changes in density in the ceiling or wall and quickly locate a structural member.

There are two types of ceiling box. a surface mount (1/2″ deep bolts directly to rafter) and a retrofit universal box. This has an attachment flange that screws onto the side of rafter, then the box screws onto the flange. It is designed to be installed through a 4″ hole in a finished ceiling.

Now, about that wiring. Post back with your situation.