Diablo 3 Guide On Health Globes - Making Them Drop : Diablo 3 Guides

Blue monsters : 60% chance to drop at 50% and 0% life.
Yellow monster : 100% chance at 75%, 50%, 25% and 0% life.
A bit risky against blue, but sometimes I'm running away and see the yellow boss has like 76% hp and try to stun him and make him drop his globe to heal when I'm low. There's even marks every 25% on the life bars to make this easier.
Got this from the Brady Guide.

Also, Health Globes dropped from blue and yellow monsters restore 35% of your life (+ boost on equipment) as opposed to 20% from globes dropped by normal monsters.
Normal monsters only drop them at death, and the odds depends on the monster (tends to be bigger on big/hard monsters eg. 5% on Corpse Worms, 20% on Dark Berserkers, 75% on Demonic Tremors).