Alex
Your question needs to be narrowed down a bit. I will take a guess that you are looking for leather to make a sheath. This being said I will try to answer you question the best I can. First you want vegetable tanned leather since is supposed to be less corrosive to the metal of the knife blade. Depending on the blade size you usually want 7/8 oz for smaller blades and 8/9oz leather for larger blades. You can get the top line tooling leather, usually shoulder or sides with no range marks, i.e. no scars, bullet holes, or thin spots, or you can get grade 2 which has a few range marks here and there. For scabbards you can usually cut around the flaws and still come up with a nice looking scabbard. If you tool the scabard, you can hide many of the marks with the tooling.
Most leather today, at least what is cured in the US or Europe, is tanned properly and you shouldn't have to worry about whether the skin is tanned properly. If you buy leather from some other parts of the world you may ocassionally come across a badly tanned hide.
I've been doing leather work for over 40 years on and off and I can't tell you how to look at leather an tell if it is good or not. It is a matter of sight, smell and feel. I suggest you take a trip to the nearest lether store, or a shoe repair shop, (Although they are getting scarce, I assume they still exist in your area) and ask someone who uses leather on a daily basis what the good and bad look like. They can probably show you good but bad might be a problem because they probably throw it out as soon as they get it.
I hope this at least helped out a little.
Jim Arbuckle
