Look for vented loops in both the inlet and exhaust lines, with vents situated above the boat's heeled waterline. The inlet line should have a U-trap under the pump to keep it wet. Make sure seacocks for both the inlet and exhaust lines are accessible and easy to use. Manual heads are the most common and usually have double-action piston pumps. Look for a pump handle that can be firmly grasped, as opposed to a mere knob, which cannot.
Most electric heads have impeller or centrifugal pumps.
Electric motors on heads should have pressure switches that cannot be accidentally left on. The inlet line should be reinforced, to stand up to suction. The exhaust line should be certified sanitation hose, to prevent the escape of foul odors. All hoses should be accessible. If you moor your boat in an area with rampant marine growth, your head may well be colonized.