I'm no expert by any means, but I'm replying anyway...
Fabric - this one takes a lot of touching and comparing before you really get a good feel for different fabric qualities. With wool blends, for instance, get a feel for what mixes/percentages/etc. feel best and pill the least. Pilling is such a vile disease.
Stitching - I
always look for straight and "strong"-looking stitching. If it looks like it's going to come undone or break, it will. And things like loose threads or long, uncut thread ends are often a good indication that little care was taken in the product's construction or finishing.
Cut - I'm not talking about how it fits you, but whether the cut of the fabric is even. This is easiest to check out with t-shirts. For example, I have a t-shirt that I hold lightly at the shoulders and let hang in a manner than it should be totally even and flat when I lay it down. Then when I lay it down, it's totally uneven. There's too much fabric at one side that should have been cut away. And it's something that is obvious (to me) when I wear it. It fits me well, so I like the overall cut a lot, but technically it's cut rather poorly. I would love to undo part of the side seam, cut away an inch of fabric, and resew it sometime. Then it'd be perfect.
Zippers - These are so important. Zippers should be straight and should lie flat. I bought a pretty cool-looking jacket once with a very attractive polished gold zipper. It was a nice detail that kind of "made" the jacket. However, it was the crappiest zipper ever. The teeth were uneven, so zipping it was quite a struggle. Actually, unzipping was the big problem, which made it even worse. Needless to say, I returned that jacket.
Seams - Puckering = bad. And look into different styles of seams. They are not created equal.
Details - After looking at the basics, just consider the overall detail of the piece. How much obvious time was put into it? Is it a basic item with run-of-the-mill construction, or is there a wonderful sublety everywhere you look? If you're paying for designer, it should tend toward the latter.