How to monitor redo apply in a snapshot standby database

SELECT PROCESS, STATUS, THREAD#, SEQUENCE#,  BLOCK#, BLOCKS FROM V$MANAGED_STANDBY;

Sample output (abbriviated):

PROCESS   STATUS          THREAD#  SEQUENCE#     BLOCK#     BLOCKS
--------- ------------ ---------- ---------- ---------- ----------
ARCH      CLOSING               1         30   18288640        750
ARCH      CLOSING               1     232528   17645568       1637
ARCH      CLOSING               1     232550          1        342
ARCH      CONNECTED             0          0          0          0
RFS       RECEIVING             1     232559     976898       2048
RFS       IDLE                  0          0          0          0
...
60 rows selected.

Tested with Oracle version 12.1.0.2.0.
Documentation for the view v$managed_standby is here
From Oracle version 12.2.0.1 and onwards, this view is deprecated. Use V$DATAGUARD_PROCESS instead, for example

SELECT  NAME,TYPE,ROLE,ACTION,CLIENT_ROLE,STOP_STATE FROM V$DATAGUARD_PROCESS;