You need to check the irreversible block:

an action is irreversible (final) if "block_num" < "last_irreversible_block"

Check this out: https://github.com/EOSIO/eos/wiki/Tutorial-Exchange-Deposit-Withdraw#machine-readable-account-history-json

If you are inside the contract and have a reference of a transaction/action/receiving-action you can assume that these are going to be all confirmed or if for some reason they are not, your action will fails also.

You need to check the irreversible block:

an action is irreversible (final) if "block_num" < "last_irreversible_block"

Check this out: https://github.com/EOSIO/eos/wiki/Tutorial-Exchange-Deposit-Withdraw#machine-readable-account-history-json

If you are inside the contract and have a reference of a transaction/action/receiving-action you can assume that these are going to be all confirmed or if for some reason they are not, your action will fail too.