Go into the training arena. You can enable a Lead Computing Gunsight that will show a green + pipper. That is the impact point of the weapon you are using at the time (wether its rockets, bombs, 20mm, 30mm, 37mm, 40mm, .50 or .303).
To save yourself some time, up from A1 and bomb the town nearby rather than going to the divebombing range at the upper right of the map. You wont see the bomb exploding, but the explosion isnt really what you need. Just release when the + gets over your chosen target (I use either the church or the big round fuel tank).
You can do this to get a feel for when to release, how the plane looks and feels at the release point and what to do after you release so you dont end up giving away proxie kills.