Can I ask in case with ghosthawk that has 2 turrets, what this command should return?

For coding convenience (and the hope that this gets also fixed in A2/OA) I would suggest just checking one turret i.e. the 'main' turret. That would be the exact turret which would be affected by 'addWeapon' or 'moveInGunner'.
Returning false for Vehicles w/o such a 'main' turret, would be OK; alternatively one could add a new Command which returns the 'canFire'-Value for each Turret via a Turret-Path-Array (i.e. [true (or false, if there is no 'main' turret), [false, true]] or [true , [true, true]] as in your Example.)
Maybe one could create a new Ticket after that one here (hopefully) gets fixed.

Btw.: I think you pointed out the exact Problem behind the Coding of this Command!
Because of the undefined Information which Turret should be considered (it seems to be stored as a Property of the Vehicle) 'canFire' returns false as a non-existing Turret isn't a Turret. Boarding the Main Turret Seat changes the Value to the Main Turret and let's 'canFire' work properly even after disembarking.
There seems, however, be a twist when disembarking from non-gunner seats (see "Additional Information")...

Personally I see no use in a command that only checks one turret but not the other and I don't quite understand the use of canFire command on a vehicle with many gunners and turrets. On a unit, yes, canStand, canMove, canFire etc, makes sense.

Also the turrets that can be damaged and rendered unusable all have hitpoints, so why not check turret damage directly with getHit or getHitPointDamage? And god forbid if some AI script is using canFire somewhere.

I am sorry if that is the Case.
But there is indeed very much Sense for such a Command. Especially in Missions of larger Scale a healthy ratio of usability and performance is vital! Caching a large Number of Strings (for a large Number of diff. Vehicles) to - maybe - check via 'getHit' (which isn't Part of A2's CMD-Pool) certainly doesn't belong to the category "reasonable" (not to speak about MP-Propagation).
But you cerainly already know a Case where it was used for a Mission of smaller Scale, too.

But BTT I think you might not understand the actual Purpose of all those 'can'-Commands: Their Routines are already used for other internal Calculation. The most Important: the AI.
So the AI switches their Behaviour exactly how all the 'can'-Commands (should) behave: if the Main Turret is destroyed, the Gunner doesn't care neither about targeting nor firing. If the Tracks are destroyed, the Driver won't care about driving anymore (currently they are disembarking the Vehicles and even a Gunner or Commander refuses to get in again). Whilst normal Gaming 'canFire' and 'canMove' both false are at least "Status Red" (due to the Pass-Through-Damage).
So these Commands are important to handle a Garbage Collector or AI Scripting.

Btw:

1. Your Suggestion wouldn't work for Tanks (I tested the MBTs), because even if the Hit Point "main_turret_hit" would be destroyed - which is listed in the config as a Property of the Main Turret exclusively - the Commander's Sight aren't moveable anymore, too.
2. The Ghosthawk seems to indeed have a Main Turret - the left Gunner. 'CanFire' would return true. I corrected my last Comment.