if you have a car that is a few years old, you should instantly be able to distinct leather from leatherette. Leather will 'wrinkle' a bit with age (even with the best of care). Silva already made the best suggestion; check the headrests, the leather will gather and wrinkle on the headrests the way it's pulled and put together. leatherette will not wrinkle or gather, it's pulled tight and does not have the leather qualities to cause it to gather (it doesn't stretch like leather).
If you have no wrinkles at all - you have leatherette. I love it, I've seen cars 10 years old with leatherette, most folks would think you replaced the original leather with new leather, because it ages and holds together extremely well. My 4 1/2 yr old leatherette looks the same as the leatherette in a new 2004 car.
Also with leatherette, you don't have to worry about the constant care that leather needs, to stay in good condition. (no leather treatments to keep it from cracking, etc etc). I also notice that leatherette seems to do better on reducing heat - you know what I mean if you've hopped into a hot car in the sun with leather seats (youch!). Leatherette seems to stay cooler (imo), my wife has leather seats (and tint even) on her Jeep, and it's always hotter than my car to jump in and sit down in.