Typically Altums have taller bodies than other Angels. They also have what some refers to as a pre-dorsal notch or said another way, they have a mouth or snout that curls up.
Altums also tend to change the intensity of their stripes very quickly in response to other fish or to stress. Fins are very erect and don't sweep back like many other scalares. The first stripe goes through the eye, the second stripe typically fades out quickly.
Wild angels vary a bit in body shape, but the altum is distinct. There are some angels called "Peruvian Altums" but there are differences. Compare the link above to these Peruvians: