What tool should be used to check for remaining soft deposits on the teeth?

The most appropriate tool to check for remaining soft deposits on the teeth is an explorer. This instrument is specifically designed for the detection of plaque and other soft deposits, as it allows the clinician to feel for and assess deposits on tooth surfaces. The explorer's pointed tip can be used to carefully probe around the teeth and in interproximal areas, helping to identify any areas that need further attention.

While a mirror with indirect vision is essential for visualizing the oral cavity and can assist in detecting some deposits, it does not provide the tactile feedback that an explorer does. A toothbrush is typically used for removing debris rather than identifying soft deposits, and an ultrasonic scaler is intended for the removal of calculus and not specifically for assessing soft deposits. Therefore, the explorer stands out as the best tool for this particular purpose.