Fire, look which ammo counter goes down, thats the weapon fired.
Check in hangar which wepon ha how much ammo, then find the corresponding value on the ammo counter to see which counter is which weapon.
With this two tasks you can figure it out.
Spit: MGs primary, canon secondary.
Mostly the more ammo gun is primary.
Also works to look at the wing which guns loosen ammo when you fire, so you can see which gun is triggerd by which trigger. Then you can find out which gun is which line in the convergence setting.
Generally fireing all guns with one trigger might work out in some planes but in most its a bad idea. It works ok on planes with equal ammo load on all guns and equal guns.
It also works good on planes that have all guns in the nose.
Its pretty bad on planes that have lots of ammo in one pairs and few in the other pair, in this case you want to use the ammo rich guns for pings, sure shots and straffing and keep the few ammo guns so you can fire with all guns on snapshots or when you need max damage.
Its bad on planes that have verry diffrent guns, since you tend to hit with the light guns, see the pings and keep shooting at this lead/direction. BUT you miss with the canons, which would actually do the damage.
Or you shoot all guns at big distance, where the chance to hit with the canons is slim, but you waste the ammo.
When you have more than one secondary, like rockets, bombs,droptanks etc. you can cycle through the secondary weaps with backspace key and fire them off with secondary gun button. Primary gun always stays the same, other cycles between gun/rocket/bombs/droptank when they are on the plane.
You can not fire primary and secondary and rocket at the same time.