Cockroaches

Aggregation pheromone and cuticular oils, detectable before the insects are. Where the smell is strongest is where the harbourage is, which makes it useful.

Chewed corners, pinholes through foil, webbing inside a bag. The packet that failed names the pest and tells you whether it arrived with the food or found it.

Four realistic candidates, separated by size and by which shelf they sit on. Most specks in a cupboard are not droppings at all, and one test proves it.

Ground-pepper specks with blunt ends and ridged sides. Size names the species, and where you find them points straight at the harbourage you have not found.

Cockroaches do not lay loose eggs. They make a hard brown capsule, and finding one means breeding on the premises rather than a single visitor.

Nocturnal narrows it to five, and what happens in the first second after you flick the light on separates them. Only one is a reason to act quickly.

Warm motor, a tray of standing water, crumbs and nobody looking. It is the best pest habitat in a kitchen, and the first place worth checking.

Crickets, ground beetles and water bugs all get called roaches. Two checks, on the antennae and on the wing cases, separate them in a single look.