How to find out the maximum current across a voltage source(using a DMM)?
You should gradually decrease the value of the load resistance while monitoring the supply output voltage. When the voltage starts to droop you have reached the maximum current the PSU can supply.

Don't short out power supplies with your ammeter. It gives you little, if any, useful information. At the current you measure the voltage is zero and since P = VI then the delivered power is zero.



From the comments:


  Good thing I bought a 2 Mohm potentiometer then :P – cst1992


A 2 M&Omega; potentiometer is useless for testing your power supply. If it is a 1/4 W potentiometer then the maximum current the resistance track can handle is \$ I = \sqrt {\frac {P}{R} } = \sqrt {\frac {0.25}{2M} } = 0.35 \ \text {mA} \$ when you are expecting 2,200 times that current. Even if it was powerful enough, 10 &Omega; would be \$ \frac {10 \times 330}{2000000} = 0.00165 \$&deg; of rotation on a 330&deg; potentiometer. Have you got a steady hand?

Note that a potentiometer's power rating is across the whole length of the track. If you are only using a certain percentage of it then derate the power dissipation by that factor as well.