The presence of nanobacteria in the environment follows from their abundance in humans, apparent abundance in the terrestrial atmosphere and agricultural irrigation with insufficiently treated water containing human excreta. Survivability of nanobacteria and increasing evidence for their implication in disease demand for their detection in environmental samples. Unfortunately, the 60-300 nm nanocrystalline biohazards resemble "harmless" aerosols that are common in the lower atmosphere and water reservoirs. Here we propose a detector for discrimination between biocolloids and nonbiogenic objects of the same size. It exploits deviations from ideal deposition patterns, formed by drops of nanosuspensions evaporating on substrates. Variations in the crystallinity and/or shape of ring patterns, due to differences in the adhesive capacity of nanoparticles contained in the drops, provide principles to design simple and sensitive biosensors, responding to otherwise nondetectable contaminations of both drops and substrates. Patterns can be visualized on mirror-polished optically reflective substrates by a light microscope operating in the reflection mode. For instance, perfect ring structures emerged from water drops which contained polystyrene nanospheres, and uniformly dense deposition patterns formed from those containing hydroxyapatite nanoparticles. The principal difference is due to the interplay between the convective flow, driving suspended material to the periphery of the evaporating drop, and attraction between nanoparticles and substrate, immobilizing them upon contact. Protected by an outer shell of apatite, nanobacteria are predicted to behave completely different from polystyrene (and most other nanospheres): in water drops with contact angles 0 <= alpha <= 90 degrees those landing around the center of the area wetted by the drop will tend to remain fixed, instead of being moved by the convective flow. The biosensor could allow the identification of nanobacteria in precipitation, water reservoirs, or filtered soil, by facilitating their localization on substrates by high-resolution imaging techniques, which are applied complementary to analytical, spectroscopic, and culture methods.