The Original Recoil Spring Test
Here’s the correct way to do it:
1. Clear the pistol. Check it. Close the slide on an empty chamber.
2. Point the muzzle in a safe direction and pull the trigger.
3. Do NOT release the trigger. Continue to hold it back.
4. Point the muzzle up into the air to true vertical.
5. Pull the slide back; and, while riding it, gently, with your support hand very slowly allow the slide to move forward. At some point it will suddenly stop OOB.
6. Now, very slowly start to lower the muzzle towards true horizontal. On many, but not all, Glock pistols the sooner your slide snaps shut, under its own power and all by itself, the stronger your recoil spring is considered to be.
7. If your slide doesn't snap shut at, or about, the 45 degree point, then, it's usually considered to be weak.
Like I said, in order to get an accurate indication from this test you've got to really know your particular Glock. Sometimes brand new Glocks have failed this test; but, in my opinion, this failure was due to something other than a supposedly weak recoil spring. (There's lots of different ways for a new Glock slide to bind to the frame.)
Still, I know my Glocks; and this test remains valid for me and for my use. It might be of good use to you with your Glocks, too.
ADVISORY: I am NOT a Certified Glock Armorer! (I'm only that armorer which Gaston Glock has forced me to become.)