What do you mean by testing skins? If you want to test out a theme present inside your themes folder before activating on a live site (probably while you are redesigning the theme) then you might try out this plugin: http://wordpress.org/extend/plugins/theme-test-drive/ I do not use thesis but I use similar complex framework i.e. genesis and it worked fine with genesis when I was redesigning a site. So, probably would work on thesis too.