To be honest I don't think there is a way to know if some coins are really lost, because the owner could retrieve his private key one day even if he is not hoping it anymore.
I also second this, as along as a coin in this regard it s sent to a valid address that’s a valid private key. Even if the owner of the private key actually losses that key it is still not a lost coin to me but rather an unmoved coin, that can coincidentally be moved should the private key is found. An exception of sending a coin to a valid address and it should be considered lost will be when that address is time lock for a thousand or million years and that still mathematically is not considered lost again.
The only way to consider a coin lost is when it is sent to an OP_RETURN output, this script is locked and can never be unlocked. The coins here are even said to be removed from supply. So I think they are qualified to be called lost coins