Methods and apparatus for detecting the presence or absence of unwanted metal deposits on a substrate holder of an electroplating apparatus are described herein. In various embodiments, a plating sensor is used to detect unwanted metal deposits. The plating sensor may be mounted relatively far away from the area that it measures (e.g., the sensor target area). For instance, the plating sensor may be on one side of the electroplating apparatus (in some cases mounted on a drip shield), and the sensor target area may be on the opposite side of the electroplating apparatus. In this way, the plating sensor can measure across the electroplating apparatus. This placement provides a relatively deep depth of focus for the plating sensor, and provides some physical separation between the plating sensor and the electroplating chemistry. Both of these factors lead to more reliable detection results.