The moment I know I need a weekend bag is the Friday morning when I'm trying to leave the house with a laptop, a pair of trainers, a change of clothes and a book I won't read, and the bag I'm holding only wants one of those things. A proper weekend bag takes the lot without you sitting on it to do the zip up, and it does it on a shoulder you can still feel by the time you get there.
This is a train platform and car boot bag, not an airport one if you're flying cabin bag only, because most of them won't pass the sizers and you'll end up repacking at the gate like an idiot while people queue behind you. It comes into its own from early autumn through to spring, when you need something that can take a jumper and boots without complaint, and it gets harder going in high summer if it's a dark leather or canvas that holds heat and makes you regret the long handle against bare skin on a hot platform. Don't pair it with anything too delicate on top, a soft tote or a structured handbag worn at the same time, because you end up juggling straps and looking like you're moving house rather than going away for two nights. It also doesn't love being crushed into an overhead locker or a boot full of shopping bags, structured bases dent and stay dented.
Get the handle drop right before anything else. Hold your arm as if you're carrying it loaded, not empty in the shop, because a bag that sits nicely on your shoulder with nothing in it can cut in once there's a weekend's worth of stuff inside. Check the shoulder strap length too if there is one, it needs to sit across the body without the bag swinging into your hip on every other step, and if it's not adjustable, that length is fixed for good. Fit problems to watch for: a base too narrow for its height, so it tips over the second you set it down, and handles that are sewn rather than riveted, which fray a season in.
While you're still in the shop, open it right out and check the inside has at least one pocket that zips, because a phone and keys loose in a bag that size will vanish under a jumper for the whole journey. Check the zip pulls are big enough to grab with cold fingers on a platform, and give the strap a proper tug rather than trusting the way it looks on the hook.
A weekend bag that can't survive being sat on a train luggage rack isn't worth the shelf space.