Drive a screw into it. This is far easier to do with a carpeted floor since the screw head (and hole) is not as noticeable. Be aware not to have any screw heads sticking up anywhere when done. They do also make special screws that will snap the head off once sunk. Thus leaving no major visible trace, but you could use normal screws as long as you got them flush.
Try sinking the screw down on or near a squeaky spot in an attempt to tighten it up. You'll need that screw to hit a floor joist as well to have something for the floor surface to tighten to. Sink one screw and test. If it didn't help, back it out and try elsewhere nearby. It helps if you can identify the joist pattern for your floor.
You could also try from below the floor assuming access was available. But in that case, I'd also use caulk/glue from there too to help quite things down.
I would not do anything from above for a finished wood floor. Too easy to mess it up and your landlord would charge you for the damage. It is potentially do-able with finish nails, skill and careful precision (which I lack).