They prefer to nest in moist environments. Conduct inspections in the evening or early morning. They are more active at night, so you're more likely to observe foraging activity and find the nest. Knock on wood near suspected nest sites. Look and listen for activity.
Carpenter ants sound like crinkling cellophane.
Check frass piles for materials such as wood, insulating, plastic, etc. Frass material will help indicate specific nest locations. Look for plastic vapor barriers under mulch beds. Ants trail or nest under the plastic film. Follow foraging ants carrying the food particles.