The first clue that there is a counter sniper operating in the area is if the snipers around you start getting picked off. The harder clue to spot is if you don't see the name of a known counter sniper getting a score or being scored on. If you don't see a certain name for a while, then that person is heading your way. This is a big reason the good counter snipers will not knife the snipers, it tells the other snipers their exact position.
A good method to draw a counter sniper into your area is to fire a few quick shots, like you are engaging, then switch to pistol and move to a spot 20 yards away where you have good cover and a view of your old spot. When the counter sniper gets to your old spot, you have a bead on him. Another thing to do is to pull back after every few shots, and scan the immediate area, first without a scope, and then with a scope or binoculars. This will at least let you know if someone is approaching. Make sure to move often, and more often if a counter sniper is operating near you. Each time you move to a new spot, keep an eye on your old spot, to see if the counter sniper is checking it.