I think you've missed my point: how do you know the wallet is HD?
I don't know with 100% certainty. I only know the wallet is
most likely a HD wallet as that is what
1) readily allows for the type of transaction in question to take place
2) is used by the most common wallet providers where one retains control of their private key (Electrum, Mycelium, Coinomi, blockchain.com, Exodus, etc).