Good question! You use an IV calculator.
This should be simple since you have just started the game. Assuming you have not trained it at all, an IV calculator would be a great idea!
Here is Serebii's IV calculator.
You can leave some parts out if you are not able to get to them (like the Hidden Power from Anistar City) but try and fill as much as you can. For EVs, just take 0 for all of them, assuming you haven't trained it at all. If you have trained it, try remember all the Pokemon that you did defeat with it and check their EV yield here. Add up all the EV yield for each stat and enter it! As long as you do a reasonable job remembering what Pokemon you did and how many EVs you got off them, the IV Calculator should be reasonably accurate.
Hope I helped. :)