Stand two canvas bags next to each other and you'll spot the difference before you've touched either one. A good one holds its shape, sits square on a shoulder, the handles sewn flat rather than just stitched on as an afterthought. A poor one slumps the second anything heavier than a paperback goes in, and the base starts to dish out into a sad little pouch within a fortnight.
The honest appeal of canvas is how little it asks of the rest of what you're wearing. It sits happily against jeans and a jumper, looks right with a linen shirt in summer, and doesn't fight with anything bright because the fabric itself is usually plain enough to stay in the background. I've worn mine with a smart coat more times than I'd admit, because a scruffy bag under tidy clothes somehow reads as deliberate rather than careless.
Canvas ages the way denim does, which is to say well, as long as you don't baby it. It softens, it fades a shade lighter at the corners where your hand grips it, and a bit of that looks good rather than tired. What doesn't look good is a dark tide mark from a leaking water bottle, so wipe spills off straight away and let it dry flat rather than shoving it near a radiator. Most will take a gentle hand wash if things get properly grim, but check the label before you do anything with a washing machine, because some have leather trims or metal fixings that won't thank you for it.
This is a fair weather bag in every sense. It's at its best from spring through early autumn, for the beach, the market, a day where you need to carry a book, a bottle and a cardigan and don't want to think about it. In proper winter rain a heavier canvas can go a bit damp and smell faintly of wet dog if you don't dry it out, so I keep mine for the months when a sudden downpour is the exception rather than the plan.
In the shop, pull the handles upward hard, the way you would lifting a full bag off the floor, and watch where the stitching sits. If it's attached only to a thin strip of fabric rather than reinforced at the seam, that's where it'll give first. Check the base too, press your fist into it, a good one will have some structure rather than folding flat like a tea towel.
A proper canvas bag is one of the few things in a high street wardrobe that looks better for being used.