How do I tell when I need to water my lawn?

You shouldn't wait until you "need to water." Doing so on regular basis will mean brown grass.

Instead, get into a regular schedule. Unless you are in a very arid area, the best thing to do is water deeply (like a lot) once a week, maybe twice during heat waves if there is no rain. Less often if there is a lot of rain. Don't water a little every day, you encourage lame root growth, which makes the grass more sensitive to heatwaves and dry spells; and it's wasteful.

Watering deeply is best because it allows the roots to grow deep, and in the long-term saves water and is better for the health of the lawn. Try to keep the lawn at 3-inches high (looks messy at first, but looks great during summer. This requires less water, makes it less sensitive to strong sun, and helps to kill out weeds, because the height of the grass makes sun less likely to get to weed seeds/sprouts.

Sprinkler systems, for example, waste water and since they are only on a short time daily or every other day, encourage shallow root growth. And, most still go on when it is raining. What a waste.

The easiest method for a beginner would be called "footprinting"

This method is so simple.

Walk on the lawn. It will make imprints where you walked. If the footprint dissapears within an hour or so, the grass is Ok. If the grass stays matted down and does not rebound up at all, time to water the lawn. You will also get used to the look and feel of when your lawn needs water if you use this method.

PS. My wife just walked in and gave me another answer after peeking over my shoulder.

She said, when your wife tells you too.

It got a chuckle out of me anyways.

good luck to you.

Use a screwdriver to test if the soil has adequate water and moisture. You should be able to drive the screwdriver 3-4" into the soil easily. If not, you need to water your lawn.

Rule of thumb: 1"-1½” of water/week. How to measure this? Measure the output of your sprinkler by putting a pan out that's at least an inch deep. Stop watering when the water level reaches 1" in the pan.

When you do, watering before six AM is best.

when it has not rain in a while and if it looks dead then ya that's when you should water it