There are 4 simple indicator :
1. Whether you're asked to store 12/24 mnemonic words
2. There's option to restore your wallet only with 12/24 mnemonic words.
3. Depending on wallet application you uses, there should be icon which indicate you're using HD wallet/not.
4. There's option to export master public key/master private key on your wallet.
it is worth adding that these may not cover all cases and it may not be true in all cases either.
for example bitcoin core is a deterministic wallet but it does not use mnemonic so there is no 12/24/... words to back up, only a seed that the user won't see in GUI. (i think point 3 is the only thing that works in this case)
or some web wallets like Blockchain.info used to have a "recovery phrase" while they weren't deterministic at the time. it looked like a mnemonic but it was for something entirely different.
I do not know much just in learning stage so, clearing my doubt. Blockchain.info was a non deterministic and I read that passphrase also introduce risk of loss when both of these disadvantage are there in Blockchian.info why were or are users using it. Does it have some positive side as well?
Sorry, if I have miss understood.
please note that non-HD wallets aren't unsafe, it is just that HD wallets are safer because you only have to rely on your RNG once when you create the seed. but with non-HD wallets each time you want to create a new private key, you have to call the RNG again and that is not desirable.
as for why people use it, as TryNinja said it is for convenience and lack of knowledge.