IV Calculator

If they are 3-4*, you can get them to lvl20 and then go to the unlock potential screen, where you can see their lvl1 stats of the next rarity. After that, you can just use the calculator for that rarity.

If they are 5*, you have to level them up fully, sadly :(

No,you can't if its a 5* units.However,if its a 1*-4* and between level 20-40,you can check IV by going to promote tab,since you will get all lvl 1 base stats.

If the Unit is already lvl 2, then you can check the IV by leveling the unit to lvl 20, and then go to the unluck potential screen without unlocking it. In this screen, you see the new stats on lvl 1 on the right side. It´s best to unequip the weapon in this case, beceause the equiped stats option on the calculator is for the + weapon which you dont get shown on the right side of the screen.

This method is good for every unit who is lvl 20 or above. If someone is lvl 39 you can even lvl it up to one lvl and calculate the final stats instead of the first ones.

If a unit is already 5 Stars, then you dont have any other choice then to lvl it up to lvl 40 to see the last stats.

^^^ What they said. :)

I'd like to add that with 5* units, you can sometimes start figuring out their IV, or at least their boon, when they're around 35-37ish.

For example, if I've got a Lv36 Sword Lucina, and her Spd is already at or above 36, I've definitely got a +Spd Lucina.

It's not a sure fire method, but it can help sometimes. :)