The Mirror dinghy is designed with a lot of built in buoyancy, but owners need to check the tanks are airtight, otherwise they could prove ineffective when the boat capsizes. An annual buoyancy test or inspection is a Class rule for those who race.
To check the tanks out, you need about 1m of tube, say garden hose, which needs to be a snug (i.e. airtight) fit in the bung hole. Then, with any hatches in place, blow one good breath (individual side or aft tank) or two good breaths (bow tank or combined aft & side tanks) into the tank. Hold your finger over the end of tube, wait 20 seconds, remove your finger. The tank should "blow back" strongly . If it does not, then the tank is leaking. You need to find the leak(s) and fix them and re-test until the tank "blows back" strongly. If you want a less subjective method, you can always make up a buoyancy tank pressure gauge such as a Class Measurer would use.
If a tank has more than one leak, the worse one will show up first and will need to be fixed before it's possible to detect the next worse one.
For those with tanks without bung holes (mostly GRP boats), you will need to buy a hatch cover, and modify it by drilling a hole and gluing in a bung hole fitting. It's a Class rule that each tank has to have either a bung hole, or a hatch.
It's best to test in a garage etc. with the doors closed so you can hear the hiss of the air as it escapes (if you have a hole). Washing up liquid plus a bit of water brushed on can help find the leak. Most likely places for leaks are around fittings (like the bung hole fittings), badly taped joints, hatches (check the "O" ring is present and in good condition) and where the beam fixed to the topsides, which supports the decks, passes through (oversize) holes in the stowage and aft bulkhead. Another possible souce is between the top of the forward bulkhead and the underside of the foredeck.
You can get leaks from some wierd locations. For example on a wooden boat, if the two panels of the foredeck don't meet perfectly under the foredeck butt strap, you can get a 'tube' formed by these 3 parts and the top of the mast web. This tube can go from the front tank and leak air into one of the cuddies and the leak could be 150mm or more from the forward bulkhead. You can get something similar on the aft transom if there is a gap between the solid wood doubler and the plywood aft transom panel. In this case the leak could let air escape along the top edge of the transom, or around a transom scupper.
One Bell/Ferranti GRP boat I tested had the aft tank connected to the front tank.
Once the tanks are airtight, you should find that they remain airtight year after year.