There's generally a more pronounced grain to the Tex. I understand that Roser leather used to have that grain as well, but over the years it tends to flatten out.
Certain colors are easy to spot--black and red, for example. Also, if it's original leather, there are always creases to varying degrees, even on the most pristine examples.
I'v also noticed that the piping on Tex can discolor and lighten over the years. This happens on the places most exposed to UV like the headrests.
Oh and one more thing -- the early Tex is the easiest to spot. The seat insert has a totally unique pattern that is completely different from the familiar leather pattern. The transition happened during the 250 SL. I'll let the 250 experts comment further.