Infrared detectors with cutoff wavelengths of ∼ 1.7 μm have much lower sensitivity to thermal background contamination than those with longer cutoff wavelengths. This low sensitivity offers the attractive possibility of reducing the need for fully cryogenic systems for YJH-band work, offering the potential for "warm-pupil instrumentation that nonetheless reduces detected thermal background to the level of dark current. However, residual sensitivity beyond the cutoff wavelength is not well characterized, and may preclude the implementation of such warm-pupil instruments. We describe an experiment to evaluate the long-wavelength sensitivity tail of a 1.7 μm-cutoff HAWAII-2RG array using a thermal blocking filter. Our results suggest the possibility of measurable red sensitivity beyond ∼ 2 μm. Ongoing improvements will confirm and refine this measurement. The thermal blocking filter offers the prospect of warm-pupil NIR instrument operation, which is particularly valuable for cost-effective and efficient testing systems: it has facilitated NIR detector characterization and will enable crucial laboratory tests of laser frequency comb calibration systems and other NIR calibration sources.