Let there be an address
How can I understand if it's a SegWit address or a MultiSig address?
Until a P2SH-P2WPKH UTXO is spent and the redeemScript is exposed, a P2SH-P2WPKH address is indistinguishable from a non-segwit P2SH address (such as a non-segwit multi-signature address)
That's because a P2SH address is still a hash of the script, regardless of whether the script is an embedded P2WPKH script or plain old script script. So you can't 'decode' it back into what the script is, you have to wait until the script itself is made public when redeeming the output.