[Plugin: W3 Total Cache] How to know it is working or not?

Here is my testing site: http://fr3x.us/

Change all option to Disk Exchanced/Disk and nothing new:

<!-- Performance optimized by W3 Total Cache. Learn more: http://www.w3-edge.com/wordpress-plugins/
Served from: fr3x.us @ 2010-08-04 04:09:17 -->

Yes, i ticked to this box: Deselect this option to disable all caching functionality.

just changed all option to opcode Xcache and preview, here is result:

Warning: xcache_set() [function.xcache-set]: xcache.var_size is either 0 or too small to enable var data caching in /var/www/fr3x.us/wp-content/plugins/w3-total-cache/lib/W3/Cache/Xcache.php  on line 40

@playgod: thanks! my site is running fine now!

my xcache.ini locate at /etc/php5/cgi/conf.d/xcache.ini cos i run Nginx webserver

do i have to edit php.ini too? because i remember that when i installed xcache, i added this code to bottom of php.ini file:

[xcache-common]
extension = xcache.so
[xcache.admin]
xcache.admin.user = "mOo"
; xcache.admin.pass = md5($your_password)
xcache.admin.pass = ""
[xcache]
; ini only settings, all the values here is default unless explained
; select low level shm/allocator scheme implemenation
xcache.shm_scheme =        "mmap"
; to disable: xcache.size=0
; to enable : xcache.size=64M etc (any size > 0) and your system mmap allows
xcache.size  =                64M
; set to cpu count (cat /proc/cpuinfo |grep -c processor)
xcache.count =                 4
; just a hash hints, you can always store count(items) > slots
xcache.slots =                8K
; ttl of the cache item, 0=forever
xcache.ttl   =                 0
; interval of gc scanning expired items, 0=no scan, other values is in seconds
xcache.gc_interval =           0
; same as aboves but for variable cache
xcache.var_size  =            64M
xcache.var_count =             1
xcache.var_slots =            8K
; default ttl
xcache.var_ttl   =             0
xcache.var_maxttl   =          0
xcache.var_gc_interval =     300
xcache.test =                Off
; N/A for /dev/zero
xcache.readonly_protection = Off
; for *nix, xcache.mmap_path is a file path, not directory.
; Use something like "/tmp/xcache" if you want to turn on ReadonlyProtection
; 2 group of php won't share the same /tmp/xcache
; for win32, xcache.mmap_path=anonymous map name, not file path
xcache.mmap_path =    "/dev/zero"
; leave it blank(disabled) or "/tmp/phpcore/"
; make sure it's writable by php (without checking open_basedir)
xcache.coredump_directory =   ""
; per request settings
xcache.cacher =               On
xcache.stat   =               Off
xcache.optimizer =            On
[xcache.coverager]
; per request settings
; enable coverage data collecting for xcache.coveragedump_directory and xcache_coverager_start/stop/get/clean() functions (will hurt executing performance)
xcache.coverager =          Off
; ini only settings
; make sure it's readable (care open_basedir) by coverage viewer script
; requires xcache.coverager=On
xcache.coveragedump_directory = ""