I would buy CO2 specific check valves, I like brass/stainless steal ones. However, the ones used for air you can buy at any pet store work fine, they degrade over time. I would buy them for a quick fix and order some nice ones in the near future.
You can check for leaks after the regulator by just putting everything under water. If you have enough slack in you lines, you can put your bubble counter under water in a cup or small pitcher. If you see bubbles, you have a leak. My diffusers make noise though that could sound like a leak. Not all do, one of mine is actually quite loud. It could sound like a leak if I didn't know it made noise on it's own.