High level clouds are those that take up residence in the highest reaches of the troposphere with a base of no less than 16,500 feet ranging to upwards of 45,000, however the average height is generally accepted as somewhere in the range of about 25,000. As that estimate is based on observation from the ground (Against which comparison to a fixed object is difficult) and pilot reports (Pireps) which are far more accurate, there is a little room for argument on that number. Identifying the high clouds is fairly easy in comparison to the middle clouds and even low clouds which are closest to us on Earth. Each has some distinguishing features you will quickly be able to pick up on to make you look like a pro.
Aside from cloud height being the first easy identifier of a high cloud, high clouds will appear white, not gray. This is due to the fact that they are composed of ice crystals which reflect the light rather than liquid forming around condensation nuclei which allows a darker appearance. Furthermore, high clouds follow wind patterns which are generally different from those of the lower level clouds nearer the surface. High clouds like their sisters at the low and mid-level can be broken down to three basic types: cirrus, cirrostratus, and cirrocumulus, however within each there is some further variety.
Cirrus and cirrostratus are so similar they can be lumped together almost. Cirrus clouds appear as very long thin white sheets in the sky. When measured against their length, they have a very small range of vertical development as compared to any other cloud. Due to fluctuating wind patterns they often appear to be collections of hundreds of small clouds grouped closely together, however it is actually one cloud which is constantly in flux being "broken" and re-formed almost constantly. While one are is "repairing" it will seem as if another is breaking. Cirrostratus are much the same but distinguished by being even more widespread taking up far more of the sky. The way these cloud types are distinguished within their family is by thickness and how broken or spotty they appear in development.
The definitive way to distinguish cirrus clouds from cirrostratus however is by precipitation. Cirrostratus does not produce any. Cirrus can prodcue precipitation although it is uncommon, most often it is virga which is rain which dissipates before reaching the surface or comes down briefly as large droplets.
Cirrocumulus clouds will also appear as a brilliant white with spotty development, but will alos appear to have "mini turrets" much like the middle cloud 8 accas. These turrets which are points of vertical development make this type of high cloud stand apart from cirrus and cirrostratus right away. Cirrocumulus clouds do not form precipitation which is another identifying point whic is why they are known as fair weather clouds. These clouds are identified within their family based on thickness and breaks, or how "full" they appear.
Identifying the high clouds is only a challenge in that they are often obscured by lower level clouds. On a fair weather day however they are easy to pick out the sky and distinguish from one another. To tell them apart look for certain key factors like precipitation, vertical development (thin, thick, or turreted) and how well collected or full the clouds are. Further distinguish cirrus an cirrostratus by noticing the actual size of the cloud, or how much of the sky one cloud takes up, and you will find it is a piece of cake!