Is it possible to preview site before updating the DNS?
 

Is there any way to view and check a site without having updated the DNS to point to the ispconfig server?

A different, in my opinion more flexible and permanent solution is to make a wildcard DNS entry for your server (i.e. *.preview.server.yourcompany.com) and add an alias domain to your site.

Thank you very much for your very obvious and very helpful suggestion :)