You can't.
IVs are determined randomly and there is no way to find out exactly. However, some NPCs in the games rate your Pokemon and comment on some of its 'good stats'. Generally these are the stats that your Pokemon has more IVs in in.
Alternatively, you can use Psypokes IV Calculator for exact IVs but that's a pain in the neck.
If you need them for determining Hidden Power Type, the games have an NPC for that too.
