I'm not sure that this will work
If the voltage goes outside the design limits, the PSU will go into foldback and cut out.
i.e. all you can do is keep increasing the load until the o/p voltage suddenly drops to zero and measure the resistance at the foldback point From the resistance, you can deduce the current in the knowledge that the voltage won't vary by more than a small amount over the linear range
You could always measure the current using a Hall Effect sensor, but unless you know somewhere that's got one, they're not cheap to buy and the above will give you just as accurate a result