If you want to confirm your zFS is running in sysplex mode, you can see following message in the syslog.

IOEZ00617I zFS is running sysplex SysLevel with interface level IntLevel.
IOEZ00350I Successfully joined group IOEZFS

And if you want to confirm OMVS is running in sysplex mode, you can issue following commands:

F BPXOINIT,FILESYS=DISPLAY

If OMVS is not running in sysplex mode then you can see following message

BPXM106I UNIX SYSTEM SERVICES WAS NOT STARTED IN SYSPLEX MODE.THE MODIFY FUNCTION CANNOT BE PERFORMED.

else you can see following messages (sample from manual):

BPXM027I COMMAND ACCEPTED.
BPXF040I MODIFY BPXOINIT,FILESYS PROCESSING IS COMPLETE.           
BPXF242I 2006/10/05 13.30.41 MODIFY BPXOINIT,FILESYS=DISPLAY,GLOBAL
SYSTEM   LFS VERSION ---STATUS--------------------- RECOMMENDED ACTION  
SY1        1.  9.  0 VERIFIED                       NONE                
SY2        1.  9.  0 VERIFIED                       NONE                
SY3        1.  9.  0 VERIFIED                       NONE                
CDS VERSION=  2        MIN LFS VERSION=  1.  9.  0                      
DEVICE NUMBER OF LAST MOUNT=      10                                    
MAXIMUM MOUNT ENTRIES=     500   MOUNT ENTRIES IN USE=       9          
MAXIMUM AMTRULES=           50   AMTRULES IN USE=            0          
MAXSYSTEM=                   8          
SYSTEM PERFORMING ZFS RECYCLE                                                  
  (Since 2006/10/05 13.30.36)                                           
  SY2                                                                   
ACTIVE QUEUE                                                            
  RECYCLING               EXCLUSIVE

https://www.ibm.com/docs/en/zos/2.5.0?topic=command-controlling-zos-unix-system-services-zos-unix

See also you should confirm XCF status:

D XCF
D XCF,SYSPLEX,ALL
D XCF,COUPLE,TYPE=BPXMCDS
D XCF,GRP,SYSBPX,ALL

https://www.ibm.com/docs/en/zos/2.5.0?topic=dc-displaying-cross-system-coupling-facility-xcf-information-xcf

Note:
Confirm MODE: MULTISYSTEM-CAPABLE or not. If not, non SYSPLEX share (MODE: MONOPLEX) mode is used.