Ornate and Nile monitors are often confused. Although they have somewhat similar markings, they are very different animals, and they are now recognized as separate species. There are many simple ways to tell them apart, however.
Perhaps the easiest way if you are now to monitors is to look at the tongue. A Nile monitor has a dark blue or purple-blue tongue. An ornate monitor has a pink tongue. Any alert monitor will flick its tongue, it should not be hard to entice a healthy monitor to show you its true colors by waving your hand in its direction.
Above, Nile monitor tongue. Below, ornate monitor tongue.
A second method is to examine the rows of spots running down the back. Nile monitors have around 7 bands of spots between their shoulders and hips. Ornates have around 5 bands. The spots of an ornate are generally larger and fewer, their markings bigger and bolder than the fine and delicate patterns of the Nile monitor.
Above, the bands of spots of a Nile monitor. Below, the bands of spots of an ornate monitor.
Third, ornate monitors are a much heftier lizard than the relatively slender Nile monitors. This is particularly evident in the head; while Niles have a robust head for a monitor, that of an ornate is positively massive. The body, limbs, and tail of the ornate monitor are generally stockier, as well. With experience, you will be able to tell these two species of monitors apart with a casual glance.
The three pictures above are of Nile monitors, the three below are of ornate monitors.
As hatchlings, the bodies of the Niles and ornates are not yet fully developed, so pictures are included below of both so you can get practice distinguishing the hatchlings.
The above pictures are of young Nile monitors, the pictures below are of young ornate monitors.