An actual measurement is required to find the accurate value of the internal resistance of the DC power supplies.
However, it can be calculated using the load regulation specification.
For example, if the load regulation is specified at 2.75 mV when the current changes by 30 A (the amount of current change when the load changes from no load to rated load), the internal resistance is: 2.75 mV/30 A = 0.09166 milliohm (mΩ) = approx. 0.1 mΩ.