An excellent test recommended by Watchguard:
A very good one:
https://testmy.net/SmarTest/combinedAuto
and another:
https://sourceforge.net/speedtest/
Here’s the first one we saw which was HTML5 only, no Flash or Java:
Here’s a commonly used one which requires Flash:
And another which uses java:
http://www.speakeasy.net/speedtest/
If you are checking this for wifi, we do recommend that you check for and rectify wifi channel congestion as a next step.