There's no way to check the exact number of EVs a Pokemon has in Ruby, Sapphire, and Emerald. However, if you are just trying to max out your Wailmer's HP EVs, you can talk to the effort ribbon girl next to the Vitamin Seller in Slateport Market. If your Wailmer is the lead Pokemon in your party and is maxed out on all EVs (you'll have to train another value in this case as well), it will receive an effort ribbon. If you're EVing for a smaller amount, provided you know Wailmer's HP IV (Find out here), you can plug the level and IV into Showdown! and toggle the EV bar until it matches your Wailmer's total HP. This isn't entirely accurate unless you have an IV of 31, though, so you might have to estimate the IV sat as well.
Sorry that it's so tedious. I hope this helps!