If the bushes cover the barrel, it will not fire. Usually if the bushes don't cover the gunners sight they will fire, if it is patchy coverage it will usually fire, it is mostly obscured it will not fire.
2 ways to check it, fire your machine gun, if it fires usually the main gun will fire, the other is test the main gun. Even then if the enemy gv is at a different angle you may have to start up and move. Hope this helps some. Also remember in most of the tanks the gun sight and the barrel are offset.