I just designed a two stage miller compensated operational amplifier, I am not sure how to measure it's maximum output current, Can you give me a precise definition of this current?
The precise definition of that current is whatever the manufacturer means. Usually a manufacturer will limit the output current so that a designer won't destroy parts by asking too much -- but in the case of power op-amps, or other specialty op-amps, that may not be the case.
The two most likely definitions of "maximum output current" are "this is the most you'll get with the output shorted to one rail or another", or it is "don't ever exceed this or you'll let out the magic smoke".
One way you could measure it would be to put the op amp in a voltage follower mode. Then put a current source on the output and slowly turn it up from 0mA. Don't go over the short circuit current rating defined in the datasheet.
If you need more current than the output you can use a booster circuit like this one:
If your only using a single sided supply, then you only need one half of the output stage.