Questions about 2 to 3 prong installations

I have an old house (about 60 years) and a few of my plugs are still the old
2 prong. I'm about to replace them with 3 prong outlets, and I have a
question or three.
What's a good and sure way to ensure I have a good ground with the conduit
and boxes? I've already had an electrician repair a loose connection in the
conduit where a GFCI failed to operate, and this got me to thinking that
there may be more of these in this aging house.
One idea I had was running an extension cord from an outlet with a known
good ground to the box I'm working on, using my multimeter to check for a
solid ground connection. Of course, I'd check for voltage first, so I don't
burn the thing up. Is this a reliable way to check for a good ground?
I do know this house has no Romex wiring, and all the internal wiring was
done to code, at least way back when the wiring was done.
Thank you for your help.
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Since you have to ask these questions, my only advice is get a licensed electrician to do the work.
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John G
I should have been more clear.
While the GFCI did work using it's built-in tester, it failed when using one of those testers that you plug in and push the button to trip it.
The way I see it, I have two choices. I can either run a ground wire from the panel to each plug, or I can use the conduit as the ground. The former would be a severe pain in the butt, and if going that route is significantly better (safer for people as well as electronics) then I'll bite the bullet and have my electrician do it. If the latter is acceptable, I can do it myself easily.
What's your take?
Right now I'm using those adapters which attach to the screw (connected properly). Would using the tester I mentioned earlier on these things be wise?
I run a lot of electronics, such as computers, off these lines, so I need them functioning well.
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I appreciate your advice, and it may very well come to that. However, I'd like to know anyway, as this has been gnawing at me for some time.
Few things are as frustrating than going through the hassle of meeting a contractor to do a job that takes 10 minutes, leaving you to think "I could have done that!"
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This is because external testers require a 'grounding conductor' to develop a current difference. The internal test does not require a grounding conductor to work. Nor does the GFCI require a grounding conductor to work properly.
It is permissible by NEC to wire three-prong outlets downstream of a GFCI *without* a grounding conductor (the third prong). The GFCI provides for electrical safety. In this situation, *all* the outlets *must* be labeled that they have no grounding conductor. Many GFCI outlet packages come with the labels inside for just such use.
As I said, if the first outlet in the 'string' is GFCI and all downstream outlets are properly fed from the GFCI and labeled correctly, the third prong doesn't have to be connected to anything. There are some circumstances where conduit might be used, but without inspecting the whole installation, I wouldn't rely on it.
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