What would be best is to use the -E argument to smbclient and redirect 2>/errorlog from the command line. You can then check this file to see if any errors occurred.
Warning, the first line is always the Domain=......... so you may need to strip that line out.
Something like this:
smbclient Hostname -A authfile -E 1>log 2>errorlog <<-EOF
get foo
EOF
In the errorlog you should find something like below, your log file will be empty
Domain=[Hostname] OS=[Windows Server 2008 R2 Standard 7601 Service Pack 1] Server=[Windows Server 2008 R2 Standard 6.1] NT_STATUS_OBJECT_NAME_NOT_FOUND opening remote file \foo