Slick is a dirt track that has lost its moisture — a hard, dusty, gray surface where tires skate instead of digging in. Grip drops sharply and the fast line usually moves up.
NASCAR’s glossary calls a dry or slick track one that “holds little to no moisture, which results in a dustier surface,” and describes slicking off as the surface getting slicker through the race until it takes on “a shiny, gray appearance.” The tells show up before the lap times do: the cushion shrinks, the color lightens, dust hangs in the corners, and cars start spinning the tires where they used to drive off. The general progression is move your line up and smooth it out, bleed a little air to grow the footprint, then free the car up on cross so it will still rotate. Watch the class ahead of you — if they’re hunting the top, your feature will be slicker than your heat.
Grip leaving is the most predictable thing about a dirt night, and the racers who adjust for it before the feature are the ones still on the lead lap.