You cannot tell.

Sure, you can just calculate it, but it's different for nearly every Pokemon, as nearly everyone has different base stats. Take these 2 stats: 200 Atk, 400 Atk.

With a negative nature, 200 Atk turns into 190, meaning 10 Atk, for 40 EVs are needed.

With a negative nature, 400 Atk turns into 360, meaning 40 Atk, for 160 EVs are needed.

It's different for each base stat, basically.

The formula for it would be:

(Stat * 0.1) * 4, rounded up the nearest whole number divisible by 4.