I just wanted to post a few tails of JUVENILE Harlan's Red-tailed Hawks since the tail can be the clinching trait for identifying juvenile Red-tails as Harlan's. However, they are not always as distinct from the other races as adults are, and can cause a bit of confusion, especially the light-morph juveniles!
What makes a Harlan's tail? Well, often the dark bands on the tail are thicker than on other races, the dark and light bands are more contrasting (but sometimes just the opposite), sometimes the banding is wavy, and each tail feather has pale blobs on the inner part of the tips creating a "spiked" look. This is not to be confused with a thin line at the tail tip, which can look like a spike of sorts that many Westerns show. Also, some juvenile Harlan's have mottling throughout the tail similar to that of adults. This is more common on light-morph juveniles, which are often different that dark-morph tail patterns in NOT showing the white blobs. Be aware that the tails of some juvenile Harlan's are nearly indistinguishable from other races, which makes ID tricky. Another trait to look for is the uppertail covert pattern. The uppertail coverts on light-morph juveniles is often mottled unlike the barred, spotted, or plain upper tails of most juvenile dark Harlan's (and most juveniles of other races).
Here are some examples -- a picture is definitely worth a thousand words. I have hundreds of examples, but these photos are a good cross section, showing every one is a bit of overkill. I was going to write an article on the subject, but it's just easier to show it here, and I think this post stands alone as a good reference for birders.