How do we know the coins in the address are actually burned?
Because the address/transactions sent to it hash to a long string of zeroes.
You can check this link for more about this type of burn address:
http://earlz.net/view/2014/10/22/0340/provably-spendable-altcoin-burn-addressesOur burn address is in that list, and will work for any altcoin of the same version.
For example, the same address we used to burn Bowscoin has been used for several different altcoins, and none of them has ever been spent on any blockchain. Just among the coins listed at cryptobe.com's explorers, there are 5 coins which have used the same version burn address from that list:
http://cryptobe.com/address/B4T5ciTCkWauSqVAcVKy88ofjcSasUkSYU