Accuratly at least level 50. Level 100 to be sure what they are, which is what IV battles are for.
Battle someone in a level 100 battle check your pokemon stats when your done just flip your game off.
When i breed pokemon tho and i get say an Adament which is what i want, i fly to the training area near stark mountain and level them up to at least level 10 for me it takes 2 pokemon so it doesnt raise your IV thats only like 4EV's at the max so ur IV's wouldnt go up. Then i check there IV's and it will give you a range like 20-30 or something. I toss the pokemon with anything 0-10 and if its 10-30 i keep them and do a level 100 battle to see its real IV's