A good outdoor table has some weight to it before anything else. Not so heavy you need two people to drag it off the delivery truck, but enough that it doesn't skate across the patio the first time someone leans on it to get up. Run a hand across the top and it should feel finished, not sticky with sealant or rough in a way that's going to snag a tablecloth or a bare forearm come August. Pull a chair up to it and there should be enough room for your knees and a drink without anyone's elbow ending up in the salad.
Check it over the day it arrives, because this is the stuff that's easy to miss in a showroom with perfect lighting. Look at how the legs meet the top, whether that joint is screwed, welded, or just glued and hoping for the best. If it's metal, look for the spots that are going to rust first, usually the feet, and see if there's any kind of coating or if it's bare and waiting for weather. If it's a glass top, think hard about whether you actually want a surface that shows every water ring and bit of pollen from April through October, because wiping it down twice a day gets old fast. Wood needs to be a type that's meant to live outside, not furniture that was pretty in a catalog and sad after one real rainstorm.
In a real room, or rather a real yard, the table sets the whole mood of the space around it. A long dining table with a bench on one side and chairs on the other sits well against a fence or a row of planters, something solid behind it so it doesn't feel like it's floating in the middle of the lawn. A smaller round one tucked near the back door works with two mismatched chairs and nothing else, no fuss. A low coffee table version needs nothing but a couple of chairs that recline and something cold nearby, which is honestly the whole point of owning one.
Small balconies and starter apartments do best with something that folds or stacks, because nobody wants to store a four seater dining table in a one bedroom closet all winter. Bigger yards and actual patios can take the dining size without crowding anything, and that's where a table earns its keep, hosting the same crowd that wouldn't fit inside for Thanksgiving. Warmer climates can leave a table out year round, which still strikes me as faintly unfair.
Next time you're looking at one, check the underside of the top first, that's where the cut corners hide.