4. There's option to export master public key/master private key on your wallet.
Clue number 4 is the best, at least to my mind. All (private key:public key) pairs are derived from one master key pair. That is what an HD wallet is by definition. By contrast, the nondeterministic wallet contains random pairs of private and public keys, and hence it cannot have a master key.
I agree this is one way of verifying and even the 3 point I felt very worth while as I could check before Signup or buying whether it is HD or not. @ETFbitcoin Thanks a lot.