As shown in the picture, you can dig into the soil and look at how deep the dry layer on top reaches. Here, about one inch has dried out 24hrs after the last watering. Note that this soil is not covered with mulch. While plants are becoming established or while seed is germinating, more watering may be needed. But, even during this period, you don't want to drown your plants or make your seed rot in cold soil.
Plants are over-watered if the soil stays muddy or puddled.
Plants are under-watered if they appear to be wilting. An automatic timer is a helpful convenience, but you will still want to get out in your garden from time to time to make sure your plants are getting the right amount of water. One year, I set my timer in the hot days of August. So, rather than counting inches of water, just get out in your garden. If it's too muddy, cut back.