Terran Missile Turrets have detection and can attack Observers. Ringing your base with Missile Turrets keeps the Observers outside the edge of your base.
Assuming that you don't have Science Vessels, the Terran detectors are
So if you're looking for a passive defense, missile turrets. If you're willing to go active, any of Ravens, Ghosts, or Scanner Sweep. Ravens will passively detect but not attack. Ghosts will attack once they're revealed but not passively detect. Scanner Sweep is an active detection that needs to be paired with offense.
Scanner Sweep is most helpful if you think an observer might be waiting somewhere. E.g. in the path out of your base. Then you can move a few units (e.g. Marines) into the area where the Observer might be, Sweep, and the Marines will kill the Observer while it is visible. If you do this more often than you leave your base to attack, then they won't know if you are attacking or just killing the Observer.
If you send a Raven with your attacking army, then the army will kill Observers as a matter of course. You can also have Ravens scout ahead of you for Observers before advancing your army.
In terms of graphics settings, I've found it to be different on different computers. There's a sort of shimmer where the Observer is, particularly if it's moving. You have to sort of explore and find what makes them stand out. This may be easier to do in the campaign, where it is more likely that you can find an Observer and not care if playing with the graphics settings makes you lose. Or in a solo game for the same reason.
Turning up the brightness tends to make hidden things easier to see in general.