Even if your pyrotechnic devices (flares) are within the expiration date printed on each flare, you can only presume that 50% of them will actually work properly.
This is a sensible rule of thumb for any small vessel venturing out of protected waters. The U.S. Coast Guard minimum requirements do not take into account the often damp and exposed stowage conditions of pyrotechnics on small vessels.
Although you can never know for sure whether the device will work properly until you try it, you can make some precautions to increase your odds. Start by checking your existing flares by pulling off the striker top. If you find beads of moisture inside, you can probably rest assured that they will not work. Next, place your NEW flares in a waterproof, zip lock container. If available put a packet of silica (the kind you find in electronic equipment boxes to absorb moisture) in the bag with the flares.