Step 3 - Verify Minification
To verify that minification is working correctly:
Test A
Visit the url: http://YOURMAGENTOWEBSITE/lib/minify/m.php?f=/skin/frontend/default/default/css/print.css (be sure to include your domain name in the URL)
This test should return the print.css file in a condensed form in your browser.
Test B
Open /var/minifycache (via FTP) to inspect the file system. You should find new files starting with minify_ which have been written to /var/minifycache.
If both tests return the expected result, minification is working correctly and you can move to the next verification test.
If either test does not return the expected result see the suggestions below:
Permission Settings (Test A and B)
Check that you have the correct permission settings for the cache directory. Check permissions on /var/minifycache. Change your permission settings to allow the web server to write files.
Note: Permission requirements differ from server to server, but most often permissions 755 or 775 will work. If in doubt, check which permissions are working for your main index.php in the Magento root folder.
Garbled Output (Test A only)
Check whether the output of Test A is garbled. Garbled output could mean that CSS files are being compressed twice. Most often, this can be eliminated by turning off zlib compression for this folder. Edit /lib/minify/.htaccess and follow the additional instructions given.