Crabgrass tends to germinate when soil temperatures remain around 50-60F degrees for several consecutive days. Visual cues are limited, but watching for weeds that germinate in pavement can help, since pavement is heated up quicker and remains warmer compared to soil. Do not aerate, mow very low, or heavily de-thatch after applying preemergent since it may reduce effectiveness.
Crabgrass is tough to combat even when using pre & post emergent killer. Keeping a lawn healthy and vigorous, especially during dry periods, is one of the best defenses. Staying vigilant is a must when controlling such an aggressive weed.