You should do a search on the various fora, this issue has been discussed several times in the past. There is no single definitive indicator because the rare LTM Sonnars were made from the (slightly less-rare) T-coated Contax RF (&, quite possibly, Contaflex TLR) versions &, like all Sonnars, those lenses underwent various cosmetic changes (changes in shape of various parts, different metals & finishes used in barrels, etc.) over their production lifetime that have no strict connection to any serial #s. IMHO, the most definitive way to tell is to disassemble the lens & look @ the rear of the optical element; if genuine, you will see 6 digits engraved along the rim of the rear element that should match the last 6 digits of the serial # marked on the front bezel. If you're already a Contax collector or know 1, another "quick & dirty" way to authenticate is to compare the lettering & markings on the front bezel & aperture ring w/those from a contemporary wartime Sonnar in Contax RF mount. Even then, there's nothing to stop a dedicated forger from adapting a Contax RF lens or using parts from 1 & putting it in a Jupiter barrel--after all, that was basically the process by which the real LTM Sonnars were produced during WWII. So it's like authenticating any old object: you have to look @ the whole thing in question, including its provenance, & compare w/the real thing.
To the best of my knowledge, the presence of the 2 "ears" on the aperture ring does *not* automatically mean the lens is fake, as almost all pre-WWII & WWII Contax RF Sonnars (both f/1.5 & f/2 versions) have the ears. I have a genuine 5cm/1.5 LTM Sonnar & it has the ears. Finding a lens without ears might mean it's genuine (albeit an unusual variation on the standard Sonnar), but the presence of the ears doesn't mean it's fake.
