You do have a way of knowing: if your DMs are going to a semi-trusted relay and you have to AUTH to it in order to read them (as you should under NIP-17) that relay may inform you about who has read them, when and from what IP address. If we implement

GitHub

nip4e: decoupling encryption from identity by fiatjaf · Pull Request #1647 · nostr-protocol/nips

this is inspired by MLS, but much simpler, and definitely not trying to be a group communication system, but only a way for users to encrypt things...

that will also kill the attacker ability's to read anything in the first place unless he wants to reveal himself by registering his bogus device or publishing new encryption keys.