Adidas shoe boxes feature a label on the one side. Check that the style (ART) and PO (#) numbers match those found on the label inside the shoe.
From the front you will see the pear shape bulge of the outsole. Also check that the stitch line which starts under the toe cap runs at an angle and not straight.
The sockliners will have an Adidas Yeezy logo, make sure the printing is solid with sharp edges. Note the antimicrobial fabric, it looks like a very fine weave.
Yeezy Boost 350's have a sonic welded seam in the insole, make sure it is not lifting. Also note the short section of heavy stitching joining the insole to the upper.
Check a full 36 point step-by-step guide on goVerify.it