Thanks to Paul Reeves for this shot.American Kestrels are actually smaller. They have pointed wings like all falcons, and when they circle, their rusty coloured tail often shows in the sun. Similarily the colour on their backs can sometimes be seen when they circle as well. Often the malar mark can be seen on their face. All indications you are looking at an American Kestrel. In flight you may see them go after a dragonfly to eat it, another good indication its a Kestrel.
Sharpshinned Hawks by contrast have rounded wings, and a square, often appearing notched tail. Their series of flaps and glides is very distinctive. The smallest Sharpshinned Hawk can be the size of a Kestrel, but in general they are larger. Both adults and juveniles have banded tails. Sharpshinned Hawks are aggressive, they will not only attack smaller birds or birds of equal size, but also larger birds of prey too. If there is a small bird attacking something, there is a good chance its a Sharpshinned Hawk.
This time of year, Sharpshinned Hawks often begin to circle. When they do, watch behind, below or above them, for a 2nd or even 3rd Sharpshin. They are often circling waiting for a nestmate. In level flight, check infront of them or behind them, you may have missed a bird ahead. At our site they like to pop up over the Poplar trees along the side of the bluff. Checking there often is a good idea.
Something that can help is that often American Kestrels fly early in the day and also late in the day. At around 10am the pre-dominate bird when the 2 species migration cycles overlap will become Sharpshinned Hawks. They will fly until the early afternoon and then more Kestrels will start to appear. Of course the time of day cant be your only criteria for which bird is which but its something to keep in mind, when you are attempting to ID birds almost instantly. In your head you can think Sharpie and look for a field marks to confirm or deny this.
Remember noone is right 100% of the time, especially on first glance(trying to ID when birds are far away). The most important thing as Colin tells me is that you get the ID right in the end.
Here you can see the pointed wings of the American Kestrel on the right and its "Christmas Tree Lights" along the trailing edge of the wings. You can also see the smaller size, relative to the Sharpshinned Hawk. You can see the square, notched tail of the Sharpshinned Hawk here too. While the wings appear to be abit pointed on the Sharpshinned Hawk, they are considerably more rounded. Thanks to Ann for this shot.
Here we can see the Sharpie and the AK in a glide. The Sharpshinned Hawk(below) clearly has much more rounded wings with "fingers" at the wingtips. In contrast you can see the pointed wings of the American Kestrel and again its considerably smaller overall size and girth. Thanks again to Ann for this shot. Please note all 3 of these photos were taken and not made, great catches by both Paul and Ann of these 2 species virtually side by side and begging for comparison.