You could always just get it to take some damage. Perhaps an AoE attack? Then watch it's health percentage.
Then all you do is use math to find out how much Max HP it has.
Quetzalcoatl.Zubis said:
»1500-1700 I think.
This I believe, At least for Ifrit.
I know Ifrit took somewhere in the ballpark of ~1,600 Damage From Pil and was left with about 2-3% HP left or so.
Assuming it took 98% health away:
1600 = .98x where x= Max HP
So that's ballpark 1632ish HP.