Read the datasheet carefully (and know how to interpret
the way things are expressed).
Some op amps (esp. older medium-voltage, 30V bipolar
types) have explicit short circuit current limiting and
as long as you respect junction temperature limits
(or cause the temp to remain within) you can just
short the output indefinitely and measure. Such
parts ought to have a min (for drive) and max (for
reliability) limit-pair.
Other op amps may spec output short circuit current
as a -minimum- value to be provided, but have no
limit to the maximum other than the output devices'
on-resistance (or, for current-mirror outputs, the
internal bias setpoint). These may or may not be
guaranteed to survive long term short circuit /
overcurrent operation.
Maybe pull output I-V curves on an expendable
test sample, and do it twice to see if you drifted
anything.