Select fastcgi as php mode in a website and enable the suexec checkbox. Then add a file test.php into that site with this content:
Open test.php in the browser, this should create a new file "newfile.txt" in the same directory. If this file is owned by the web user and client group of this website, then suexec works. If the file is owned by www-data or a similar apache user, then suexec does not work as expected.