how do I find out which glow plug is faulty?

Question

my 2004 Renault megane has fault codes p0670 and p0380 come up on diagnostics, p0670 is glow plug control module and p0380 is glow plug heater circuit, battery is only at 8.4volts could this be the cause or is it a glow plug or power supply to the glow plugs? if its a glow plug how do I find out which 1? please help me lol