I understand that a lawn may need to be dethatched periodically so the roots of the grass can reach into the soil (instead of into the thatch layer). How can I tell if my thatch layer is getting too thick? (Preferably before the lawn starts to look bad.)

I understand that a lawn may need to be dethatched periodically so the roots of the grass can reach into the soil (instead of into the thatch layer). How do I tell if my thatch layer is getting too thick? (Preferably before the lawn starts to look bad.)