If you are grilling jumbo prawns with the shell on, if will take a little experimentation and closer observation to see when the jumbo prawns are grilled to perfection, but it isn't hard. You'll have to rely less on looks and more on the texture and feel of the grilling prawns to judge when the prawns are grilled completely.
The trick is to grill the jumbo prawns until they become firm - and not any longer. You won't be able to see the flesh change in color inside the shell of the grilling prawn. You will be able to gently squeeze them against the grill and to test their firmness by feel.
Grill a jumbo prawn or two as a test to get the feel for it, and you'll soon be an expert.
Remember that the grilling jumbo prawns will continue to cook for a short time after you remove them from the grill, so ideally you take the prawns off the grill just when they are about 90% to 95% cooked and then as you get the prawns from the grill to the plate they finish that 5% to 10% and be at a state of perfection when the prawns are eaten.