What I most commonly see on the iPhone 6 with liquid damage is C5202_RF (the capacitor on the bottom side of the motherboard next to the wifi chip) will be shorted to ground and prevent the phone from turning on. You will have to carefully remove the motherboard from the case and inspect the bottom side to ensure that it visually looks okay (and if you have a multimeter handy diode mode measurement should be ~0.4 Volts).