This is unfortunately common. The board is treated as an intractable whole, the attitude is that you buy a new product rather than repair the broken one, because it is cheap to make, expensive to pay someone to repair it, hence the schematic never sees the light of day outside the manufacturer.
Sometimes you can work out what is what, sometimes you can't. Sometimes you can take an educated guess as to the general area of the fault. It's made worse by multilayer boards where you can't see the traces.
Random component testing can work - I'm guilty of doing it, and found a faulty part, though not the real cause of the fault. I have a mains over ethernet box and a freeview box waiting for me to have the time for exactly this method.
1-3-2-4, if you are sure it is the smps which is at fault, many people say you should replace all the electrolytic capacitors even if they check ok with a meter because their esr goes up as they age.