What is the best way to determine if 24 Hr Grub Killer was effective in killing the grubs ? Is it when the dying grubs come to the soil surface?

The grubs would not come to the top of the soils. However, you will notice the grubs starting to curl. This would indicate that decomposition has begun. The way 24-Hour Grub Killer works is by shutting down the grubs digestive system, causing them to starve to death. They may still be moving around for up to a week, however, they are not actively feeding. Jordan