Interfacial engineering is a proven strategy to enhance the efficiency of perovskite solar cells (PeSCs) by controlling surface electronic defects and carrier trapping. The trap states at the "top" interface between the perovskite and upper charge extraction layers are experimentally accessible and have been extensively studied. However, the understanding of the unexposed "bottom" surface of the perovskite layer remains elusive, due to the lack of selective and non-destructive tools to access buried interface. Here, a new spectroscopy technique is introduced that monitors nanosecond to millisecond dynamics of trapped carriers at the buried interfaces by combining optical trap activation by infrared light with surface photovoltage detection. Applied to various PeSC architectures, this method reveals that most interfacial traps reside between the perovskite and hole transport layer, suggesting a predominance of hole traps (e.g., cation and lead vacancies) over electron traps (e.g., halide vacancies) in the studied PeSC systems. The proposed new approach separates interfacial carrier-loss contributions from the top and buried surfaces, providing design insights for achieving high-performance PeSCs through interface optimization.