To use the GPS, you need to have the GPS item in your inventory.
As for getting your bearings without map contacts, if you have the GPS you can RCtrl-M and see yourself on the mini-map. Short of that you'll need to rely on your Mk I Eyeballs for spatial location services.
this addWeapon "ItemGPS";
Look around for landmarks, roads especially. The small boxes on the map are 100m square, so if you see that there's a T intersection 100m NE of you and you find that on the map, you'll be able to place yourself. It's difficult, but not impossible, to keep track of yourself on the map. For OA at least the map is smaller than it seems, meaning you might think you haven't traveled far but you might have.
For traveling long distances in a car use the GPS and leave yourself markers. At each intersection put a small black dot with R or L indicating which way to turn. Do this in the Vehicle Chat channel to avoid messing up the map for everyone else. That way as you're driving along you'll see the marks on your mini-map and be able to make the turns you need.
As for detecting the enemy. Just pay attention really. That part is still making me dead! I can never tell where fire is coming from and it's exhilarating! heh