How do i know wich one it is?

Thats 15 attack. Good one. Lucky

Just do more power ups until it shows the correct IV.

Normally I'd say to put it in a gym and use Gymhunter to get the stats, but that won't work here because he's legendary. Frankly, it's not that important whether you have 12 stamina 11 defense or 11 stamina 12 defense. It's the strongest mon you have, unless you have another mewtwo.

You already powered it up once right?

If so, another power up will make it level 21. At level 21:

HP 12 ATK 15 DEF 11 Mewtwo will be 2347 CP, 125 HP
HP 11 ATK 15 DEF 12 Mewtwo will be 2348 CP, 124 HP

So just do one more power-up and you will be able to figure out which combination it is. Frankly, it doesn't matter much. You have the 15 ATK. Congrats!