It's simple to prove the creation of a UTXO: You just need to provide the transaction data, the Merkle branch, and block of the creating transaction.
Then, as a full-node, it is simple to check whether the output hasn't been spent yet: You're keeping track of the UTXO yourself, and if it hasn't been removed from your UTXO set, it's still unspent.
As a lightweight client doesn't have the full UTXO set at hand, how could it make sure that an output is still unspent? I see that BIP0064 introduced a
getutxo message, but can't seem to find more about it. Is such an interface still in effect? Is there an option beyond polling a number of different nodes for a particular UTXO, or running a full-node?
Clarification of what I'm interested in: I've recently read the paper Sybil-Resistant Pseudonymization and Pseudonym Change without Trusted Third Parties. The proposed system BitNym manages pseudonyms by storing them in transaction outputs. A pseudonym stays valid until the anchoring output is spent. It is therefore of interest to check whether an output is still in the UTXO set.