It depends on timing and country. In Sweden (and in some EU countries), if a defect appears within the first 2 years, it’s presumed to have existed at delivery, you only need to show that the defect exists; the seller must prove otherwise.
After that (e.g., year 3 in Sweden), you must show it was an original defect (photos from delivery, technician/retailer assessment, etc.).
EU minimum presumption is 1 year, with some countries at 2 years.