Unfortunately there is no way to soft reset for good natures or IVs with Pokemon given to you via the Pokemon Link. This is because they are placed directly into your PC, and aren't given to you by a special NPC, like the Mystery Gift Pokemon are. The game automatically saves after using the Pokemon link, so you are stuck with whatever you are given. Like Doctor Disco said, all legendaries in Gen VI are guaranteed 3 perfect IVs, so that is at least something.
As for checking IVs, talk to a man standing in the Pokemon Centre at the Battle Resort. This is obviously only possible if you have reached the post-game. If you can't access the Battle Resort yet (it is post-game), then just use an online IV calculator, since they come with no EVs, are level 50, and you know their natures, so the calculator should be accurate enough to give you an idea of their IVs. The Battle Resort is less hassle, though.
