Leveling them up to level 100 and then using showdown (or better yet, a hidden power calculator) is a way to do it, but if you don't want to level them up at all, there are more complex methods to find their IVs.
- Challenge the battle institute. Once it levels your Pokemon to level 50, check their stats and plug it into an IV calculator.
- Check with the IV judge found in the battle resort and compare it with the results from the battle institute. (Results from the IV judge, if you do not know them, can be found here.)
- After following those two steps, use the hidden power calculator to find what hidden power it is. If the results you get do not give you a definitive answer for the IVs, then leveling your Pokemon up is the only way, I'm afraid. Of course, there is hacking, but that's a whole different and more complex topic that I'm not going to get into.
Hope I helped, and good luck!