You can't unless a. you do the online battles that raise their level and check there or you have to use the IV Checker in another town. Otherwise you'll just have to make due with guesstimates. If you're really despereate, an easy method is keep count of it's lvl 1, and then put it in day care for like a minute. It should pop up as a lvl 5-8 and should give you a much better idea what possible IVs it has.
"I've just remembered something...He was not as naive as you are. That's what made him a hero." Zero from RMZ