All concepts, explanations, trials, and studies have been
re-written in plain English and may contain errors. I am
not a doctor
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This info comes from 2 different sources so you can
compare how 1-RM may be found in healthy people.

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A 1-RM weight can be lifted only once:

Start with a weight that can be lifted comfortably.
Continue to add weight until the maximum amount that
can be lifted once is found. Trial and error is usually
used. The greatest weight that can be lifted one time
is the 1-RM max for that person for that weight exercise.

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Kramer and Fry suggest the following protocol for 1-RM
testing:

1)  Warm-up with 5 to 10 repetitions at 40% to 60% of
    your ESTIMATED maximum.
2)  Rest for about 5 minutes.
3)  Increase the weight to 60% to 80% of the estimated
    maximum, attempting to do 3 to 5 reps.
4)  Now a small increase in weight is added to the load
    and a maximum lift is tried, with maximum being able
    to do only one rep at that weight.

The goal is to find the 1-RM in 3 to 5 tries. There should
be at least 3 to 5 minutes before each 1-RM attempt.

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