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.)
According to your profile, your location is Bellefonte, PA, therefore unless you correct me, I am going to assume you have a cool-season lawn.
Thatch: Enemy of Lawns from University of Missouri Extension:
If you follow the above link you will see a photo that clearly shows what thatch build up in a lawn looks like.
Cool-Season Grasses: Lawn Maintenance Calendar from University of Missouri Extension:
You may also find the information found in the below links well worth you time:
If you decide you need to dethatch, I would recommend you aerate at the same time.
I dethatch my lawn when the layer is about 3/4" thick . A thick layer of thatch can also:
I would avoid dethatching (or'scarifying') in spring, when the grass has not yet produced side shoots - if the lawn is opened up, it will become more vulnerable to weeds. The best time to scarify is probably early autumn, when removal of the thatch will stimulate the grass to produce runners and side shoots.
See also my answer to Lawn Dethatching Tools here.
I'd go by the way it looks - if it looks like there's quite a bit of yellowing, dry, and dead grass then I'd scarify.
Alternatively, if you don't want to wait until it begins to look bad then periodically just scarify a small patch to see how much thatch is lifted. If there's a lot, the lawn will probably benefit from a good scarification.
This video shows what I mean: Lawn Care & Landscaping : How to De-Thatch a Lawn
A useful side-effect of scarifying by hand is that's also very good exercise!