1) In the Russian Church: Only tonsured nuns are addressed as “Mother”—the others are addressed as “Sister.” This is exactly the same as the practice regarding monks, who are called “Father” only if tonsured. The clue is whether they wear the “klobuk”—the cylindrical black hat with the veil—if they wear a klobuk, they are called “Father” or “Mother.” The problem with this clue is that the klobuk is worn only in church or at formal occasions—the rest of the time monks and nuns wear “skufias”—soft, usually velvet, pointy hats. Abbesses, of course, are always addressed as “Mother”—in Russian, the greeting is usually the diminutive “Matushka” [pronounced with accent on the first syllable—webmaster].
2) Only Abbesses (or in rare cases their chief assistants) can wear a pectoral cross. Laypeople should approach an Abbess for a blessing the same as they would a Priest—they bow and hold their hands, palms up, right over left—and after receiving the blessing (which the Abbess makes holding her fingers the same as when one makes the sign of the cross—not the “Name of the Lord” configuration of fingers used by priests when blessing)—they kiss the Abbess’s hand.
3) When a Priest greets an Abbess, he blesses her as usual, but they kiss each other’s hand, exactly as two Priests meeting (or two Bishops) do. Abbesses stand in a throne and hold their staff, which looks like a Bishop’s staff, except it is made of wood.
4) In her convent, the Abbess is the Rector. The Priests who serve in the convent do nothing without her knowledge and blessing. The serving Priest bows to the Abbess when beginning the services, and he censes her before anyone else. The Abbess is commemorated by name at all the major litanies and at the Great Entrance. In many ways, the Abbess is given respect by the serving clergy similar to that given to Bishops present at the service, except that she is censed only three times, not three-times-three.
5) Abbesses can enter the altar at any time. In larger convents, certain nuns are appointed by the Abbess (with the approval of the Bishop) to enter the altar to maintain it and the vestments of the clergy, and even to assist the serving priest if no male altar servers are available—but it should be known that other nuns cannot enter the altar. Nuns appointed to help in the altar are usually chosen from those who have been in the convent from a young age.
The Russian approach is usually rather easy: if they’re wearing a pectoral cross, you can get a blessing from them. Unless, of course, they’re outside of the church and not wearing their cross…