1 - Read the tags. Will this plant do well in the space you want to put it?
What type of light does it require?
How tall is it going to get?
Is it a perennial or annual?
What degree does it say it is hardy to?
If a blooming plant, how long is the bloom time?
2 - Bugs
Are there small gnat type bugs flying around it when you pick it up?
Are there scales or oval brown type bugs on the stems or under the leaves
Are there small white fuzzy spots on the stems or under the leaves
Are there spider mites or small webs on the back sides of the leaves, very tiny
Are there aphids on the plant? - usually small black or green bugs with legs
3 - Check the leaves and stems
Are there a lot of broken stems, torn leaves or brown foliage?
4 - If it is a blooming plant, are the flowers blooming or still buds?
It is better to pick a plant that has more buds than open flowers, it will extend the flowering season for you to enjoy when you bring it home.
5 - Is the plant soil dry or moist?
If it is a succulent - the soil should be more dry than wet. Most box stores over water these types of plants and instantly kill them.
Any other type of plant can have moist soil and be fine
If it is too dry, the foliage will be wilted or falls apart when you touch it.