Brown patches are one major clue that you have grub worms.
According to Cornell University, just a few grubs aren’t a problem. To determine if grub worms are the main cause, dig up a square foot of grass where you find the discoloration. If you have 0-5 per square foot, then it’s not an issue. If you have 6-9 grubs per square foot, then you might want to consider treating your lawn. This is especially true if raccoons or other predators are digging up your lawn to get at the grubs.
Regardless, with 10 grubs or more per square foot, you need to treat.