A friend recently purchased a few 1 oz gold Maples. On receiving them, I helped him measure the diameter with a micrometer, and found all of them were wider the published diameter. I contacted the Royal Canadian Mint via telephone and was surprised at their answer to my inquiry. Here is what they said: Yes, the RCM receives a Seigniorage fee for minting the coins; BUT they refuse to guarantee ANYTHING regarding the thickness, or diameter of the coins. They will only assure you of the weight. Their attitude is that you should have their coins assayed to assure they are in fact gold. I also emailed them, but they did not respond. Seems to me, the message of the RCM is when buying Gold Maple coins: buyer beware.
Well i believe that the best way to test is through its weight i am sure there is standard weight if the difference is little with its standard size
But this is not practical.I would suggest to get the coins from a standared seller so that you can claim for any difference.