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We demonstrate a novel, multi-purpose optoelectronic characterization technique to quantify light trapping and photoinduced charge generation and extraction in photovoltaics and other multilayer thin-film optoelectronic devices. The technique measures the photogenerated current created via the selective evanescent coupling of incident light into each of the guided modes of an optoelectronic device. In analogy to the internal quantum efficiency commonly used to characterize photovoltaics (the ratio of photogenerated electrons extracted from the device to photons absorbed by the device for normally incident light), we define the guided-mode internal quantum efficiency (GIQE) as the ratio of photogenerated electrons extracted from the device to the photons absorbed by the device for a specific guided mode. We complement the measurement of GIQE with computational modeling to calculate the electromagnetic field distribution within the various layers of the device, enabling us to separate the contribution to the GIQE of the absorption in the photoactive layer from parasitic absorption in other layers. By separately quantifying the quantum efficiency of each guided mode, this technique enables improved optimization and design of optoelectronic devices, including photovoltaics that utilize waveguiding and light-trapping. Additionally, since the electromagnetic field of each guided mode has a unique spatial distribution within the photoactive layer, this technique also provides insight into the spatial distributions of charge-carrier extraction, regions of disorder, trap states, and defects within the photoactive layer.