Across a room, a good tea dress reads as a shape, a soft fall from the shoulder to somewhere mid calf, with the waist doing a quiet, specific job. A poor one reads as fabric, a shapeless float that could be a nightie if you squint. The difference is almost never visible up close, it's visible from the doorway, which is the whole point of the style and also why it's so easy to get wrong.
What separates the two is mostly in the hand. A tea dress wants a fabric with a bit of weight and drape, viscose or a good cotton blend that falls rather than clings or crackles, because the whole silhouette depends on movement. Cheap polyester versions look fine on the hanger and then sit stiff across the hips the second you walk, like cardboard that's learned to resent you. Check the seams at the waist and underarm, not the bodice, that's where a tea dress fails first, usually within a few washes if the stitching was done in a hurry. Buttons down the front should sit flat and not gape by the second one, and if there's a self tie belt, it should come with actual belt loops rather than floating free and migrating to your ribs by lunchtime.
In the shop, do the arms up test before anything else, lift both arms properly and see if the bodice rides up or the sleeve seam digs in, because that's the movement you'll actually be doing all day. Sit down in it too. A lot of tea dresses are cut generously through the skirt and mean through the seat, and you won't know until you're folded into a chair. Check the hem length against your own calf rather than the model's, the "right" spot moves a surprising amount depending on where you're shortest, and a dress that lands mid calf on someone 5ft 9 can land at a deeply unflattering point on someone 5ft 4.
Where it falls apart is wind, hills, and anything requiring speed. A full skirted tea dress on a gusty day is a liability, and cycling in one is its own small disaster I won't describe in detail. It's also the wrong choice for anything that needs structure, a big meeting, a formal do, it reads soft and a bit retro rather than sharp. Don't pair it with anything else that's already doing a lot, a statement coat or heavy boots turn the whole thing fancy dress rather than considered, keep the rest of the outfit quiet and let the dress be the one decision.
A properly cut tea dress is one of the few things that looks better after a season of actual wear than it did on day one.