The destruction time scale of dust in the diffuse interstellar medium is estimated to be an order of magnitude shorter than its residence time. Nevertheless, dust is observed in the interstellar medium, leading to the conclusion that reformation and grain growth must take place. Direct observations of nanometre-sized silicate grains, the main constituent of interstellar dust, would provide a smoking gun for the occurrence of grain condensation in the diffuse interstellar medium. Here we employ quantum chemical calculations to obtain the mid-infrared (IR) optical properties of a library of Mg-end member silicate nanoparticles with olivine (Mg2SiO4) and pyroxene (MgSiO3) stoichiometries. We use this library as an input for a foreground-screen model to predict the spectral appearance of the absorption profile due to mixtures of bulk and nanoparticle silicates towards bright background sources. The mid-IR spectrum observed towards an O8V star or a carbon-rich Wolf-Rayet star starts to change when ∼3% of the silicate mass is in the form of nanosilicates. We predict that a 3-10% nanosilicate fraction can be detected with the James Webb Space Telescope (JWST) using the mid-IR instrument (MIRI). With our upcoming JWST observations using MIRI, we will be able to detect or place limits on the nanosilicate content in the diffuse interstellar medium, and thus potentially directly confirm interstellar dust formation.