There are a few guidelines to use when buying a plant at a nursery. The first is, if you are selecting a flowering plant, the best choice is a plant that is not in flower. For annual bedding plants, this may be nigh onto impossible. Growers know that most people like to see their flowering plants in flower- it is attractive to see rows of flowers and makes it easier to pick out colors. So, what is wrong with this? Generally, plants in flower were forced into flower for sale and are often root bound. In addition, they are more likely to be somewhat stunted in their growth, though there are exceptions: I have luck with petunias, though marigolds that look like a little plant topped by a flower ball are likely to be stunted, in my experience, and will not produce a satisfying display. Another exception is tomatoes- these plants can root along the stem, so being root bound is not as much of a problem. So pick out a plant with few flowers, small buds or no buds at all.
Second rule- when buying a plant, tip it out of the pot if you can. If there are roots snaking out the bottom drainage hole, or into another pot, try to avoid buying that plant. What you want to see are some fine roots near the edges of the soil in the pot- you want good root growth. What you don’t want to see is many roots, wrapping around and around the perimeter of the pot- this is a root-bound plant. It will probably not thrive.
Third rule: check for bugs- scales or eggs under and on the plant leaves or stems (scale bug), webbing (spider mites), a dusting of white when you gently brush the plant (white flies). This is especially true for house plants- bring one infested plant into the house, and the others will get infested too. Of course, also avoid plants with excessive wilting (infested, diseased or under watered), that are dripping unless recently watered (overwatered or water logged), with yellowing, dead leaves or leaves with holes in them. Don’t buy plants with obvious mold or fungus or mushrooms growing in the pots (they were probably often overwatered and may have root rot). Try to avoid those “bargain shelf” plants, unless you know what you are doing- they may transfer disease or may be poor performers.