It is very difficult to diagnose a fault without turning the power on.
Hoever, there are a few initial checks you can do:-
do a visual inspection of the PCB looking for any obvious damage, eg. burnt components, cracks in the PCB tracks, dry joints, etc.
A good magnifying glass is essential as some problems are hard to see. For example, my TV had a dry joint that was only visible with a Mag Glass. There was fine crack in the solder around a wire.
After that, assuming you did not find anything, you need to turn the power on and use a multimeter.
Firstly measure the power supply voltage(s).
Then measure the voltage at various points in the circuit looking for one that is not what you would expect.
It is a big help if you have a circuit diagram. Otherwise you may have to draw one which is tedious & time consuming.