Start by taking a look at the meat coverage. Whether it's baby back ribs, spare ribs or St. Louis-style ribs, you want to have plenty of meat on the bone for everyone to sink their teeth into. In regards to marbling, you want a little more fat on spare ribs and St. Louis ribs as this helps give them their flavor, but it's not necessary on baby backs that are covered in a thick coat of rub.
You also want to give the ribs a good sniff. If you catch a whiff of sulfur when you open the package, rinse the ribs in cold water. This is a normal smell that is the byproduct of processing and is nothing to worry about.
Finally, take a look at the liquid in the bottom of the package. If there is a lot of liquid, it could indicate the ribs have dried out a bit, which means you will want to marinate your ribs before you slide them onto the grill or into the smoker.