This is only necessary if your seed phrase is BIP39. If it is, then when you recover it in Electrum, you simply click the box "Detect existing accounts" and it will (probably) find the right derivation path for you.

The only way to know if you used a passphrase is to remember using a passphrase. There will be no indication from the seed phrase or the wallet it recovers that you used an additional passphrase. The only hint will be if your wallet is empty and you expected to find some balance, then maybe (but not necessarily) you are looking for a different wallet with the same seed phrase but an additional passphrase.

Recover your seed phrase on Electrum. In the box that you enter your seed phrase, after you type it it will show up "Seed Type: legacy" or "Seed Type: segwit" underneath if it is an Electrum seed phrase.