The length of the mag will tell you. Very short the 26, approx. 1" longer 19, 1 1 1/2" =17. Thats just off the top of my head. Now the longer mags will fit into the 26 just stick out a bit.
may I suggest trying to go to the glock store? try these: Magazines & Extensions - GlockStore.com
What Glock do you have? Are you wanting to get more rounds ?Or just keeping with the issued mag?