Click on Site Settings
Look and Feel section, choose Master Page.
on Site Master Page Settings---
under Site Master Page
you can find default master page name which applied to your site.
How to check out master page from SharePoint site
OPen SP site
Site settings
Page Layouts and Master Pages
search for master page
How to open that master page from sharePoint designer
Open SPD
Under All files
_catalog
Master page
find your master page
Open in Advanced mode
edit it
Save
