As this Casascius 1BTC coin is fully loaded, does this mean it holds1 Bitcoin BTC, PLUS 1 Bitcoin Cash BCC?
What is the precise methodology to check that the balances are there in full?
Yes, it means it holds 1 BTC and 1 BCH. To check the balance, you will punch in the first bits you see in the middle of the hologram on here:
https://casascius.uberbills.com/ or search it on the signed list provided on casascius.com. You can then use your address and check on a BTC explorer and a BCH explorer.