Summer weeds, like crabgrass, must have several consecutive days of temperatures above 55 degrees before they germinate. If the seed is right on the surface of your soil, it becomes warm quickly. If the seed is buried a bit deeper, it might take weeks before the soil becomes warm enough to initiate the germination process.
To determine if it is too late to apply a weed preventer to your lawn, all you have to know is how warm the soil has been for the last few days. You could use an indoor/outdoor thermometer, inserted with the bulb 1 inch deep, to measure the current soil temperature. A more accurate measurement is available through the Georgia Environmental Monitoring Network.
In Duluth for the last few days (April), soil temperatures at the two inch depth have hovered in the low fifty degree range. While a few crabgrass seeds have germinated so far this spring, many of them have not. If you spread a pre-emergent today and water it into the soil, it will still provide good weed control. By knowing the soil temperatures in your area, you can determine whether there is still time to keep the weeds at bay.