If you are in a tower,and the enemy is close enough, you already are being updated about the enemy, so when you spawn you see him instantly.
When you spawn, you are a new vehicle in the world. It can take a while before the system starts sending updates on the new vehicle to the vehicle that was already on the ground.
If you spawn to remote spawn points that are outside update range, it is then pot luck who will see who first.
How long this takes depends on how many people are in the arena.
If you wish to test, have 2 friendly's spawn at the same time same place , and observe the outcomes. Bombers would be best because of external view.
HiTech