If you didn't see an orange message that says "You have collided", then you didn't take damage from a collision. If you see the orange message, you were damaged in the collision. How severe depends on which parts your PC says were occupying the same space occupied by the other plane.
Due to lag, the positions of your plane on his FE, and his on your FE, won't be exactly the same. They will be close, but not so exact that a collision will be recorded by both FEs. At 300mph, you travel 44 every tenth of a second. That is enough of a difference to allow one guy to collide and one guy to get by cleanly.
Basically, don't try to fly through the other guy's plane. If you do, you will likely find that the collision model works exactly like it should.
