Question: How could McClane tell that the ammo in the blue labeled magazine were blanks?
Answer: If you listen, he says something to the effect of "I know I had that guy in my sights", meaning he is amazed he missed. So he checks the bullets in the magazine to see if that's the problems. Blanks are simply shells with no bullet that are crimped shut to hold the powder in. The difference between blanks and live rounds is very obvious even at a glance.
Answer: Just before the attack on the church, Grant and his team switch the red taped clips from their weapons with blue taped clips. The blue clips carried blanks and the red clips carried real ammo. After firing on a guy but not hitting him, McClane checked the clip and realised that the gun he used had blanks and that Grant and his team were involved with freeing Esperanza.
Quite apart from the fact that anyone who has spent his whole professional life with guns would know instantly that he had just shot off a magazine of blanks – no recoil.
Answer: Additionally, in the military, blue tape or blue in general is associated with non-lethal. Red tape is for danger, i.e., magazines with live ammo. A blue grenade is inert and meant for training.