I normally take first looks at the manufacturer, who makes the product you are trying to buy? Then I move on to the overall design of the item I am interested in buying, what is the material used to make the item? You also want to be observant of the colors used, ensure they are the proper shade and spread evenly in the right spots.
I like plushies, and when I get any of them they most likely will be pikachu ones, so I ensure the colors and designs are matching the official art. Then I look closely at the details, like the placement of arms, legs, facial pieces, and tail. Sometimes you'll see a funky looking sewing job on some of the products, and those are easily avoidable if you take precautions before buying. When I buy a plushie, it has to be made by the official brand of most pokemon products.