Dragon's Claw

• WWizards of the Coast2009-10-01
  Official Ruling
  The ability triggers whenever any player, not just you, casts a red spell.
• WWizards of the Coast2009-10-01
  Official Ruling
  If a player casts a red spell, Dragon’s Claw’s ability triggers and is put on the stack on top of that spell. Dragon’s Claw’s ability will resolve (causing you to gain 1 life) before the spell does.

Rulings via Scryfall