You can quickly determine if the layer your child is wearing is car-seat appropriate by placing them in the seat and properly adjusting the harness then removing them from the seat WITHOUT loosening the straps. Once out of the seat, remove the jacket or sweater, place your child back in the seat, and buckle them in, checking for any excess slack in the harness with the layer. You can best ensure the harness is adjusted tightly by again trying to pinch a horizontal fold in the strap at your child’s collarbone. It may be necessary to tighten the straps a very slight amount after removing the layer, but if there is significant slack or visible difference in how the harness is positioned on your little one, then that layer is likely too thick to safely use in the car seat.