is it possible to tell from an xpub if a wallet is single sig or multi sig?
No. An xpub is only used to derive public keys. You can use several xpub keys to form a hierarchical deterministic multi-sig wallet, just as you can with multiple public keys (but one address). Their existence alone doesn't reveal they come from a multi-sig or single-sig wallet.
While you are correct in saying that an xpub can be used to create single-sig or multi-sig wallets and you can't tell what it was used for, note that this is only the case for legacy multi-sig. If you use nested segwit, then your master public key will generally be ypub for P2WPKH nested in P2SH but Ypub for multi-sig nested in P2SH, and for native segwit will be zpub for P2WPKH and Zpub for P2WSH.