You're gonna have to look at the Base Stat for a Level 100 Deoxys, then subtract that amount from the Stat. Then divide that remaining amount by 4 and you should get how many EV's it has approximately.
So: (You're Deoxys's Stat - Deoxys's Base Stat for the Stat) ÷ 4. This is the formula.
And no, there's no other way of reducing EV's without the Berries.