How I know that Twitter is down
I begin to suspect that Twitter is down when the Update button fails to post a new tweet promptly.
I become more suspicious when browser Refresh/Reload fails to load the Twitter web page.
My suspicion is mostly confirmed when Bit.ly has a notification at the top of the page:
Twitter is currently experiencing some API issues. Posting may be unavailable.
Finally I go to IsTwitterDown.com and it says "yes" in red letters in the middle of the page.
Twitter is still down.
How long has it been down? Answer: Too long.
How long is that? Long enough for me to complete three old-fashioned blog posts!