Try a stud finder. Also, if you're ever parked in a humid area overnight, the sides will sweat but the studs won't. Take a picture when that happens for later reference.
Don Ford
2004 Safari Trek 31SBD (F53/V10 20,500GVW)
'09 HHR 2LT or '97 Aerostar MiniVan (Remco driveshaft disconnect) for Towed vehicles
BlueOx Aventa II Towbar - ReadyBrake Inertia Brake System