For Class I composite replacements, how do dentists identify where the filling is on the occlusal and differentiate between filling and enamel?

[Jono]

Assuming there are no identifiable filling margins on the occlusal surface, how does a dentist identify where to drill exactly on a molar?

Phosphoric acid etch the entire occlusal surface?

 

[honestdoc]

Usually we have magnification with full lighting. The filling to be replaced usually have numerous visible defects and fillings reflect light differently than enamel. I etch only on the surfaces to be bonded. If the filling is intact and has no defects on x-rays, then the filling does not need to be replaced (unless you're in a learning facility and working on a typodont, then look for the different light reflection).