Start Emacs with -Q, run `list-faces-display', move to a face with a box
defined (for example, mode-line-highlight), highlight the sample text
with the mouse, and move the cursor around inside the sample text.

If you're lucky, you will be able to observe artifacting, usually around
the characters "O", "Q", "R", "W", "X", "Y", and/or "Z".  If you're not,
you can usually at least observe the text being redrawn in a manner
inconsistent with its surroundings, where characters surrounding the
cursor appear to be spaced overly close to or overly far from their
surroundings.

I've attached a screenshot depicting the results of moving the cursor to
the end of the sample text for mode-line-highlight while under mouse
face, and then quickly moving the cursor to the next line.  Note the
artifacting to the left of the character "Z", and how "Z" appears to be
spaced further apart than its neighbors to the left.