What is the default font family in HTML and how can I check this?

Here is an overview of default web browser fonts: http://www.granneman.com/webdev/coding/css/fonts-and-formatting/web-browser-font-defaults/

You can also download a browser plugin to select some text and get the font name (e.g Font Finder for Firefox)

Each browser has a default stylesheet, e.g. this one from Chrome:

https://chromium.googlesource.com/chromium/blink/+/master/Source/core/css/html.css

Alternatively, just Inspect the element, and look at the computed tab.