Firefox, Safari, and Chrome all have developer tools:
Show Develop menu in menu bar
You can also right-click and choose Inspect Element
.
Check out https://www.modern.ie/
Most awesome testing site ever!
Exercise:
We'll example http://www.mesonet.info/services/barnes2.xhtml
...mostly because I know the exact resources it will access.
You can do this with any web page / site.
Exercise with Safari:
Developmenu your phone should be listed.