Do you have bare PCBs spare, as in PCBs with no components?
Start by applying power to a bare PCB, is there a short?
If yes, then you can either carefully trace every track looking for the short or you can leave the power connected and wait for the tracks to blacken and possibly go up in smoke, then you will know where the problem is.
If the bare PCB does not have a short then solder a few components on and repeat the test. Start with power related components first. Keep adding components until you find the problem.