The website sniff test: if the site has a checkbox, can I click on the text next to the checkbox to check/uncheck it?

If not, they probably messed something else up too, so I'm going to be careful with using tricky things like the back/forward buttons, passwords with special characters, etc.

For passwords in particular I seem to have developed a sixth sense for looking at a website and saying "Ugggh, I better not give them too complicated of a password," and then configuring my password manager to not use any fancy characters or make it too long.
Another good sniff test is if your name contains a special character like an apostrophe or a hyphen. If you do, then your signup experience is basically a "Van Halen brown M&M's" test for whether they've built a good website or not
Did Van Halen's Concert Contract Require the Removal of Brown M&Ms?

Did Van Halen's contract call for the removal of all brown M&Ms from bowls of candy placed backstage?

Snopes
@djoerd Yup it's one of those strange but true stories that becomes a great analogy you can use in a lot of different situations. :)