How to find which is your default master page for non publishing sites

If you are working with a non publishing site there are probably multiple tweaks to find the master page, the one i find simple (if you have designer) is

1. select each master page in the gallery
2. the one for which "set as default master page" is disabled is the one which already is one

If you are working with a publishing site, finding the default master page is quite straight forward