jQuery UI uses the QUnit test framework. All the unit tests are in
http://jquery-ui.googlecode.com/svn/trunk/tests/unit/
There's a subfolder for each plugin, and in each plugin's folder a file for each set of tests: core, defaults, events, methods, options, tickets.
Many aspects of jQuery UI need to be tested visually and interactively. A Visual test is a simple web page that excercises some method or option and allows the tester to assert visualy or interactively whether it passes. All the visual tests are in
http://jquery-ui.googlecode.com/svn/trunk/tests/visual/
There's a subfolder for each plugin, and in each plugin's folder, one file per test. These follow a standard naming convention. For example:
dialog\
default.html
dialog.html
dialog_event_close.html
dialog_method_close.html
dialog_option_modal_true.html
dialog_option_modal_false.html
dialog_ticket_1234.html