Good artificial flowers should make someone touch them to check, and bad ones announce themselves from across the room. That's really the whole test. If the first thing you notice is the flower, something is off. If the first thing you notice is just a nice bit of color on the table, you got it right.
Check the stems before anything else, because that's where the cheap ones fall apart first. Real stems have a slight irregularity to them, a little variation in thickness, a bend that isn't perfectly uniform. Molded plastic stems are too smooth and too green, the color of a highlighter rather than a leaf. Look at the base of the petals too, where they meet the stem. On a good one that join is tight and almost invisible. On a bad one you'll see the glue, or worse, feel it, a hard little bump under your fingers. Give the whole stem a gentle bend when you're looking at it in person. It should hold a soft curve and not spring back like a coat hanger.
They age fairly well if you treat them with a bit of respect, which mostly means keeping them out of direct sun. Fabric petals fade the same way a cushion on a sunny windowsill fades, unevenly, so the side facing the window goes pale while the back stays rich, and once that happens there's no fixing it. Dust is the other enemy, though it's an easy one. A quick pass with a hair dryer on the cool setting knocks dust out of petals far better than wiping, which just presses it into the fabric. Every few months, pull the stems out of whatever they're sitting in and fluff the petals back into shape with your fingers, since they do flatten and droop a little from gravity and from being jostled in a vase.
The difference between a good bunch and a sad one usually comes down to weight and leaf detail. Pick it up. A well made stem has some heft to it, a proper wire core rather than thin plastic, and the leaves have visible veining and slightly varied shades of green rather than one flat color across the whole bunch. Cheap ones also tend to use too few stems to fill a vase properly, so you end up with that thin, lonely look no matter how you arrange them. And scale matters more than people think. A single oversized bloom can swamp a small side table, while a delicate little bunch disappears on anything larger than a nightstand. They're also the wrong call if you actually love the ritual of fresh flowers, the water change, the smell, the slow droop over a week. That's a different pleasure and no fake stem replicates it.
I'd put mine in something plain and a little heavy, so the vase doesn't look like it's trying as hard as the flowers are.