Step 1: Look in the mirror, or get someone to inspect the valves as you breathe out. The silicone piece inside the valve should be moving in and out.
Step 2: Place your fingers just above the nose piece on your bare skin, to feel for escaping air. Or put on a pair of glasses to see if they fog up.
Step 3: See if you can easily put your finger into the filter under your chin. If you can, pull the mask up on your cheeks. The active carbon filter should be tight along your chin. Test again by placing your finger on your skin under your chin.
Note that with the microfiber testing slip inside the mask, it will be more difficult to breathe than without it. Once you are sure you fit the mask, you can remove the hygienic microfiber slip to reveal the active carbon filter.