The first thing to check is the silver content of the mirror. Generally, the higher the silver content, the better the product is.
The thickness of the mirror will also reveal its quality. Thin and low-quality mirrors will result to warping or distorted images. Again, the thicker, the better. At the very least, the thickness should be 1/4 inch.
If you are purchasing a set with multiple mirrors, then it's critical that you ensure that the glass comes from one source. If the seller simply packaged the mirrors from different sources, the unappealing differences may show once you set up the entire set.
When shopping for an entryway table and mirror set, don't forget to check the mirror. Most people make the mistake of just checking the table and ignoring the mirror's quality. Keep in mind that even a luxurious-looking mirror will reveal its "cheapness" if the mirror is of lousy quality.