So ive been checking the crater grotto for imposter ditto since like the game came out and haven't seen him yet but I want to be prepared for when I do see it. As most of you probably know the only iv that matters for this ditto is hp and since It would be easy enough, I might as well get ditto with 31 hp ivs. I know a couple ways to determine Ivs like iv calculators and talking to the guy in the battle subway. I also tend to just plug in the pokemons characteristics into the pokemon online team creater and determine them that way though this method can be inaccurate especially with lower level pokemon. I've also heard a statement in the past that hinted that someone could tell the dittos hp iv just by entering battle with it. Is this possible? If not whats the best way to go about making sure I get the best ditto possible?

Also if anyone is interested, I will trade for an imposter ditto no matter the ivs.