I go from recommendations. I have a little internal log, really, of places that are still good quality or have good quality things. It's usually quite expensive but still high street, so I buy fewer things or on eBay. If I hear about a good place I buy something to try out, like a pillowcase, and if it's good it goes into rotation.
So I'll buy clothes from Toast, Cos, Palava, Community Clothing, Margaret Howell and Joseph (on ebay obvs for those last two!). I'll buy Clarks boots as they are still decent, somehow. I will buy "feels like down" pillows and duvets from Dusk, but nothing else from there. Bedding from Rise and Fall (used to be Soak and Sleep and White Company but they've gone downhill). Furniture from Tikamoon or second hand. John Lewis Anyday can be decent - I have a really solid bed from that range. Waitrose and M&S still also fairly reliable for now.
There may well occasionally be decent things in other shops but who has the time to sift through all the shite. It's the same principle as restaurants: the longer the menu, the worse the food. The bigger the shop, the worse the products. In this case I do mean physical size! It's not perfect (Waitroses can be large), but it's a good rule of thumb. In general, if the store is not a human scale building, it's probably stuffed with utter junk from China. If the company is a multinational megacorp, then its products are probably 95% tat.