I'm not sure if it would power on if you don't have the fans and temperature sensors plugged in, when I test for voltage output to the logic board, I had the I/O board flex cable and power cable plugged into the logic board and one memory module, but I neve
r test turning it on with just that.
If you do try that, in case you didn't know and it does power on with just the logic board and io board, be sure to have the heat sink mounted from keeping the cpu+gpu from burning up.
Have you tried just having the ac adapter and battery removed and holding the power button for 10 secs and remain holding the power button and then plug in the magsafe cable and continue to hold it for 10 secs and then just press the power button as normal to see if it will turn on? If not, give that a try.
I know with mine, I wasn't even getting the battery to charge.
If you want to test at component level, you can start by testing the 2 chip fuses on the IO board, there will be one near the magsafe connector and one on the bottom of the IO board, the fuses are small and soldered on the board, you will see the letter F and some numbers after near the fuses.
You can also short the two contacts on the logic board that have the letters PWR next to them, and will be found in between the memory and dvd drive if the board is still in the case, if the macbook turns on, you know its the power button.