I make the hypothesis that you're talking about P2SH addresses (starting with a 3)*
Yes, I am.
Unfortunately, the scriptpubkey (script stored in the unspent output) of a P2SH can't be used to retrieve the M & N values.
The scriptpubkey contains a hash of the redeem script (the multisig script) and this hash can't be reversed and interpreted.
The redeem script will be provided in clear in the scriptsig when the utxo is spent. But that may be too late...
It's okay! Can you tell me how to find M & N values after UTXO is spent?
Providing a proof that a P2SH is legit is a good feature which should be proposed by all multisig online wallets.
It just requires you have access to the redeem script and the hash before sending funds to the P2SH address.
Hard part. It can be faked.
*: old multisig scripts don't have this "problem" since the multisig script can be found in clear in the scriptpubkey.
I didn't there was two different multisig. Can you please give more info about the multisig you are talking about? Thank you!
-MZ