The first experimental demonstration of lasing plasmonic nanoparticles in 2009 ignited interest in active plasmonic structures with optical gain. However, the understanding of lasing in plasmonic nanoparticles is largely incomplete, and even less is known about their characteristics as they are taken toward the lasing threshold. Here we present a computational method and predictions of the lasing wavelength and threshold gain for spherical core-shell nanostructures with a metal core and a gain medium in the shell. We demonstrate that light scattering provides a simple diagnostics method to establish how far a specific nanoparticle is from reaching the lasing threshold. We also show that these structures can enhance the electric field by a factor of over 1500 (at 99.9% of threshold gain) and beyond, taking biosensing with these "smart dust" nanoparticles into the single molecule sensitivity regime.