There are probably other/better ways to see what datafiles are being managed by ASM, but this is one way. The command asmcmd lives in the $ORACLE_HOME/bin irectory. In this example I have created an ASM area for my database files, control files etc. By cd’ing to the DATA directory I can see the files that are created in the group.
[oracle@beefy bin]$ asmcmd
ASMCMD> ls
DATA
ASMCMD> ls DATA
10I
control_001
control_002
data_1
data_10
data_11
data_12
data_13