There are many ways in which you can tell if your lawn has grubs. First of all, if you have lawn grubs, it may be difficult to recognize the signs, at least at first. Grubs will cause your grass to turn slightly yellow first. When they are eating the roots, then your lawn is going to get less water. Grubs will also come out at night and eat grass that is touching the surface of the soil. Grubbs have chewing mouthparts, that allow them to digest your grass, and they will turn green after they have feasted on your lawn
Another sign that you have grubs in your lawn, is if when you are thatching your lawn, the grass rips out easily. Usually. This is because the lawn grubs will eat some of the deepest roots first. And there will not be much to hold your grass in place.
If you have had a bad infestation of grubs in a sod lawn, you may have three quarters of an inch of dead sod in areas where the grubs have been. You typically don't see the spots until the end of the summer, or the beginning of the fall. By this time, it is probably too late in the year to treat your lawn for grubs. But you can be sure to treat your long for them early in the Spring next year.
A third sign that he may have grubs in lawn, is if you have gophers that are digging up your lawn, or you have birds that are picking holes in your dirt. They are not actually trying to damage your lawn, they are just trying to get lunch. And they really like eating grubs.
If you are experiencing any of these signs, it may be a good idea for you to check for grubs. To do this I usually go to the worst area of lawn, and pull up dead sod in that area or on the edge of that area, and I look for white grubs. These grubs will be ½ to ¾ inch in length. They are white, and do not look like an earthworm because they are short and stout. If you do find them, you will want to treat your lawn immediately. Hopefully, you're treating it soon enough, you can prevent them from doing any worse damage your lawn. You may also want to apply the treatment twice, to kill grubs that are unhatched and which may be immune to poison the first time you apply it.