As I recall, you had demonstrated leakage from a motor
inside the expresso machine. The conductivity function in a
meter probably will not measure that leakage. 'Kludge' the
power cord connection so that a meter will measure as little
as microamps of AC current. IOW that meter must be able to
measure tenths of AC milliamps. You must put the meter in
series with the safety ground prong.

The GFCI only trips when the motor powers on. Since all
wires are separated, then the motor would be the leakage.
Leakage in motors often means a circuit path that cannot be
measured by the DC test voltage from that meter. A better
alternative would be disassembly, cleaning, and inspection of
that motor.

To answer the risk question, numbers of amps (or milliamps)
that are leaking down the safety ground wire must be known.

fredinstl wrote:
thanks for the answers. I will go buy a multimeter, although I am not
sure
what I should do to correct if I find it doesn't do what Autotracer
explained
below.

What are the risks of using a non-GFCI outlet (not grounded as I live
in an old house) for a few weeks ?
My in-laws are visiting soon, and I'd like to be
able to offer them decent coffee.