It seems no general key for identification all Malus species. You may search for the information you need in the local floras. Also species first descriptions can be helpful. But also note that a lot of markers for identification now - not only morphological, but genetic, bichemic and so on.
Malus orientalis is a wild species of apple (Malus domestica) native to Middle East and Central Asia. Malus orientalis is ancestor of present day cultivated apple. It is primary genetic relative of apple. This wild species is potential for disease resistance in apple.It is a deciduous tree species generally standing 5-10 meters. Malus orientalis is distributed in temperate regions of Asia (Iran, Turkey, Armenia, Azerbaijan, Georgia and Russian Federation). As suggested earlier you can get information about this species from local flora.