The dash is always a good starting point. Oftentimes the shape and/or gauge location is a dead giveaway. The presence of a/c controls or the aforementioned CD player are pretty good clues, as well (although I've seen pictures of originals with stereos, they are typically mounted under the dash rather than in it).
The color of the interior (anything besides black) is a clue, too, especially if it's trimmed out like a "new" car (edging on the carpets, etc.). Although I've seen originals with red, blue, etc. interiors, they always look like they were done in somewhat of a hurry as opposed to being painstakingly hand-trimmed by Skippy the Jaguar Restorer over the course of a long winter and several cases of Guiness.
The radiator area can be telling. Some replicas have a very "upright" rad which is a dead giveaway. Also, some have "pusher" fans that are not the same as the ones installed on S/C models. I've also seen a couple of original cars that had non-stock pushers installed, but that was 15 or more years ago and I'm pretty sure most of those have been returned to "correct" form.
Some replicas with round taillights use a brand that appears smaller in diameter, in terms of lens area, than original cars.
Cars with bare aluminum fender vents are typically replicas.
Lastly, the overall shape of the body is oftentimes a giveaway, although some original cars look more like replicas in that department than a lot of replicas! A glimpse at the VIN plate or speaking with the owner is the best method, although such inquiries should be tactfully worded to avoid the previously mentioned gunfire--
