If you are wanting to know if it is doubling before you shoot it , function test it.
REMOVE MAGAZINE / CHECK CHAMBER / MAKE SURE RIFLE IS UNLOADED / HAVE NO AMMO NEAR RIFLE
With unloaded rifle , pull bolt carrier back , cocking hammer.
Allow bolt carrier to slam home.
Pointing rifle in safe direction , pull trigger (hammer should fall) , hold trigger all to the way rear.
Pull bolt carrier to the rear (still holding trigger) , let carrier slam home.
Look through top cover slot. Is the hammer being retained by the disconnector ? No problem.
Release trigger. The hammer should be released by the disconnector and caught by the hooks on the top of the trigger. Were they caught ? No problem.
If the hammer is not caught by the disconnector and / or hammer it is UNSAFE and will double.
If the hammer is not caught adjust your disconnector , as Campy outlined.
If the disconnector is properly adjusted and the hammer does not "follow through" during the function test then it should not double during actual firing of live ammo.
*********************************************************************
You r-e-a-l-l-y should function test any semi / full auto before live firing it for the first time.
I had one one MISR Maadi that would have doubled (or maybe emptied the whole magazine) from the factory. Luckily I checked it before I fired it the first time as expecting a bang and getting bang , bang , or worse bang ,bang, bang, etc can be embarassing and potentially deadly.
*********************************************************************
Once you successfully function test the rifle and are ready to live fire load one round in the magazine , chamber it and fire it (at a safe target).
Keep the trigger pulled and check to see if the disconnector caught the hammer. Did it ?
OK.
Load two rounds and repeat the test , checking after each round to see if the hammer was caught ,before realeasing the trigger.
If the rifle passes these tests it is safe.