The following example shows how to find out which server is currently the primary when deployed in single-primary mode.

mysql> SELECT VARIABLE_VALUE FROM performance_schema.global_status WHERE VARIABLE_NAME= 'group_replication_primary_member';
+--------------------------------------+
| VARIABLE_VALUE                       |
+--------------------------------------+
| 69e1a3b8-8397-11e6-8e67-bf68cbc061a4 |
+--------------------------------------+
1 row in set (0,00 sec)