There a few different types of grubs that can affect your lawn.
These include those laid by moths and those laid by beetles. The beetle variety can be easier to break the life cycle because you can kill the adult beetle, while the ones laid by moths can be more difficult as the adult moth is harder to kill.
Generally adult moths and beetles are attracted to healthy lush lawns. They will lay their young in a lawn that will give their baby grubs the best start in life. After the baby grubs have decimated an area the adults will just move onto the closest healthy bit of lawn and start again.
Often when householders notice they have a problem the moths and beetles will have already left this area and being happily munching down on a new patch of green. This is why when you look for the sign of grub life in the decimated area there is none, they have already moved on!
So, when checking if your beautiful lawn has any squatters, you should dig down into a healthy area just bordering the already eaten part and have a look there for the signs of grub life. Different species of grubs have different habits, including whether they eat the roots or leaves of your grass and where in the profile of your lawn they live. Some prefer to live in the soil, while others like the thatch layer of your turf so make sure you dig to about the depth 20cm and pull the turf apart and have a good look.
Wasps hanging around your lawn are also a good indicator of having a lawn grub problem. The parasitic wasps lay their eggs in host larvae, commonly lawn grubs. This is good for the lawn as it kills the host grub.
You can also soak the lawn area with a wetting agent. The wetting agent will ensure the water penetrates into the soil. It is a waste of time to apply the wetting agent to the brown dead area as it is likely the grubs will have moved on. The wetting agent should be applied to the edges of the eaten and healthy area and see if any grubs come up. You can also place a damp hessian bag down overnight and have a look in the morning if any are present.
Some species of grubs are also much more likely to do damage to your turf then others. This will depend on whether the species you have in your lawn occurs in plague proportions and literally eats the lawn, or whether it is the humble lawn beetle laying a few grubs in a healthy lawn and not causing any damage overall. A few lawn beetle grubs in a healthy lawn are generally no problem, live and let live. A few lawn beetles in an unhealthy lawn with a weak root structure can however cause a fair bit of destruction.
If your lawn is looking decidedly sick (and the leaf looks chewed on close inspection) there is a good chance you have one of the lawn grubs that does occur in plague proportions. These species are normally laid by moths and include web sodworm and army grub.
Sod webworms prefer fresh new grass (freshly laid), however they will choose an older lawn if it is healthy. This is because new lawn stores more nutrients in the leaf until the roots are into the ground and sod webworms eat the leaves of your lawn.