According to peter :
"Chris Lewis" wrote in message

Three-light plug or neon testers are sufficient to tell you
that a ground _exists_ (in the former, the two green lights but not
the red one light up), but not how "good" the ground it is. If
they light up appropriately, the ground is almost guaranteed to be
good enough to trip a GFCI. But it's nowhere near a certainty that
a hot-ground short would trip the breaker.

So how do you test how good the ground is?
Connect a hair dryer across hot and ground (instead of neutral) and see if
it works just as well? Obviously this test would not work on a GFCI
receptacle.

That'd work, but a 100W lightbulb on a pigtail, looking for full
brightness is a bit safer and somewhat easier to judge the result.

Either way, pull it out _immediately_ if it's noticably dim or begins to
flicker, and keep your eyes and nose open for smoke.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.