Paintings can be damaged in many ways. The canvas might be torn or punctured, it may have split canvas edges; it may have developed sagging canvas, bulges and dents. If on panel rather than canvas, you may observe splits, warps and cracks in the wood; the wood will also be susceptible to insect damage (e.g. wood worm). Even if the substrate appears sound, you may find that the image itself has areas of cracked, loose or flaking paint, lost paint, or fading.