if the question is "schnorr signature verification" and your code assumes the ID is correct, then it's just a simple schnorr signature verification against the signature bytes

if your code validates the ID is maybe not correct, you have to convert to the canonical form and derive the hash of this, so, it depends on how deep you want to do the validation

the fact is that right now on the network, the only events i've seen that are now frequently failing to generate correct IDs are zaps that seem to be associated with either mutiny or alby lightning wallets, and they are unusual events because they contain embedded JSON inside the tags

the signatures are then on these in correct ID hashes and thus they are actually invalid events

nobody is checking for this afaik but i've reported it to alby and mutiny but no idea if they have fixed it, it looks like they have, because i don't see this ID check fail anymore