Dentists usually poke around in your mouth to find cavities. When the probe "sticks" in a tooth, they look closer for decay.
Some dentists are switching to the diode laser to find and remove cavities. Decayed teeth glow when they’re exposed to the laser.
The diode laser can find cavities in teeth without fillings. It doesn’t replace X-rays: It finds decay in grooves on the chewing surface, while bitewing X-rays can find decay between and inside teeth.