A thangka is much more than decoration. Every figure is painted on a fixed proportional grid, so the size of the face, hands and seat follows rules handed down for centuries. That is one quick way to judge quality: in a careful painting the main deity sits perfectly centred, the hand gestures (mudras) are clear, and the small details like jewellery, lotus petals and flames around the halo are crisp.

It also helps to know who is shown. Green Tara, Medicine Buddha, Avalokiteshvara and the Wheel of Life are the most common subjects, and each has its own colours and symbols. If you are looking for one for practice or for your home, browse a few works side by side first. A site I found useful for comparing styles is Buy authentic thangka painting, which lists hand-painted pieces from Nepal with the deity named.