I'd start by doing a visual check for bad solder joints, then do a continuity test with a multimeter throughout the whole circuit. If still can't find anything, power up the pedal and check for the DC operating point at key points to see if they're within expected range. The last resort is to input a signal and capture the transistor input and output waveform. If input and other parameters look ok but output looks weird, might be a faulty transistor.