Field guides consistently add to the id problem by getting things wrong and not mentioning what we consider to be the best features to look for. These are:
1) At all ages Shikra has 5, sometimes 6 distinct bands on the underside of the tail. This can be difficult to see in the field but is easily seen on photos. A tip here is to try to get a photo of any suspect bird because that can be the key to identification is many cases.
2) Shikra has a flight silhouette that is somewhere between Sparrowhawk and Levant Sparrowhawk in that it has 5 distinct primary 'fingers' on a soaring bird. Sparrowhawk has 6 whilst Levant Sparrowhawk has only 4 which gives it a very falcon like silhouette.
3) At all ages and in both sexes Shikra shows a distinct dark gular stripe which is absent in Sparrowhawk.
Juvenile Shikra, Kuwait. Showing tail bars, dark wingtip and gular stripe. Copyright Pekka Fagel
Juvenile Shikra, Kuwait. Showing gular stripe and pale eye ruling out Levant Sparrowhawk which always has a dark eye. Copyright Pekka Fagel
Adult Shikra, India. Showing gular stripe and undertail bars
Adult Shikra, The Gambia. Although a washed out photo the gular stripe and tail barring are still there (honestly!)
Juvenile Shikra, Kuwait. Note the 5 primary fingers and also the gular stripe even visible on this distant shot. Copyright Pekka Fagel.