how do i find an electrical fault on my astra gte mk2 cabriolet?

i have an astra mk2 cabriolet on a g plate when i turn the key to accesary sometimes the engine management light does not come on as it should when this happens the car will not start at all but if the light does come on the car is perfect on starting this problem is becoming more frequent i really need to get to the bottom of it. its startin to drive me nuts