You can spot the good beach bag from the car park. The cheap ones sag the second you put a towel and a bottle of water in them, the straps dig in at one shoulder, and by the second week of a trip the bottom's gone grey and shapeless from being dropped on wet sand. A proper one keeps its shape loaded up, stands on its own on the ground rather than flopping sideways, and still looks like a bag rather than a sack of washing by the end of the holiday.
Stick to the ones in raw straw, faded stripe, or a single solid colour like terracotta or navy, because anything with a loud print or a trend colour looks dated the following summer and you'll be stuck with it regardless. A straw or canvas tote in a neutral shade goes with everything you'd actually wear to a beach, a linen shirt, denim shorts, a plain swimsuit, and it won't fight with a patterned sarong the way a busy printed bag will. These earn their place from late spring through to the end of a warm September, for anything from a day trip to the coast to two weeks away, and a lot of women end up using the same one for the school run or the farmers market out of season too, so it's worth choosing one that doesn't scream beach specifically. On fit, go bigger than feels sensible. You need room for a towel, sun cream, a book, water, someone else's flip flops, so anything under about forty centimetres across the top will be full after one trip to the shop for ice creams. Check the handles sit comfortably on the shoulder when it's loaded, not just empty in the shop, because an unlined straw bag with thin handles will cut in once it's got any weight in it, and look for a zip or clasp top if you're taking anything you don't want falling out when you bend down for a shell.
The real test is sand, salt water, and being shoved in the boot of a car for a week. Straw ones fray at the edges if they're badly woven, so give the base a proper squeeze in the shop, if it feels stiff and tight it'll survive being sat on. Canvas ones should come through a few washes without the colour going patchy, and the good ones still look fine after a season of being dragged across car parks rather than carried properly. A beach bag that still looks decent after being dropped in wet sand three times in one week is worth far more than one that just looked nice in the shop mirror.