cd /tmp && touch file_real && ln file_real file_hard && ln -s file_real file_soft
ls -li file_*

for filename in file_real file_hard file_soft; do echo -n "$filename "; zabbix_get -s example.com -k vfs.file.exists[$filename]; done

All three give 1. http://www.zabbix.com/documentation/2.0/manual/config/items/itemtypes/zabbix_agent claims:

"It returns 0 for pipes, symbolic links, directories, etc."