Perforce allows each file in a client to be at a different revision, making it sometimes impossible to ask the question "to which changelist is this client synced?" This script checks if all files are at the same changelist before reporting that CL.

perforce-current-synced-changelist.py

#!/usr/bin/env python

import argparse
from P4 import P4
p4 = P4()
p4.connect()

def main():
    argparse.ArgumentParser(description="Verify that all files synced to the same changelist, and sets ERRORLEVEL to the count of unsynced files.").parse_args()
    client = p4.fetch_client()['Client']
    change = next(p4.iterate_changes('-m1', "@" + client))["Change"]
    with p4.at_exception_level(P4.RAISE_ERRORS):
        unsynced_file_count = len(p4.run_sync('-n', '@' + change))
    if (unsynced_file_count):
        print ("Client {client} is partially synced to CL{change} but some files are out of sync. Run 'p4 sync -n @{change}' to see the full list."
            .format(client=client, change=change))
    else:
        print ("Client {client} is consistently synced to CL{change}"
            .format(client=client, change=change))
    return unsynced_file_count

if __name__ == '__main__':
    sys.exit(main())

Depends on p4python from https://www.perforce.com/perforce/doc.current/manuals/p4script/03_python.html