The hardest boards are the ones with no schematic, no service manual, and no forum thread. The good news is that most of them talk before you power them: you just have to look.
Start with your eyes. A strong light and a loupe over the whole board, both sides. Bulged or leaking electrolytics, brown scorch around a resistor, a hairline crack at a connector, cold or cracked solder joints, corrosion from a leaked battery or a spill. These are free findings and they solve a real fraction of dead boards before the meter comes out.
Then the power rails, in order. Find the input, confirm it reaches the first regulator, then confirm the regulator's output is in the ballpark its part number promises. A datasheet for the regulator and a multimeter are often the only schematic you need, because most failures are in the power path, not the logic.
Two disciplines while you work. One: compare against a working unit whenever a second exists, because "is 1.8 volts right here" is unanswerable and "is it the same as the working one" is not. Two: write down every measurement as you go. A board mystery is a notebook problem before it is an electronics problem.
What board is on your bench, and what did your eyes find? Post the regulator markings if you are stuck; that is usually the fastest way to a datasheet.