From what I saw yesterday, encrypted messages had just “…” as subject.
I see a “Chat-Version: 1.0” header, and “X-Mailer: Delta Chat 0.40.0 for Android” - but the values of both are likely to change in the future.
I guess doing a “contains” match on the “X-Mailer” header and look for “Delta Chat” should work, but I’m open to better ideas.