This protocol outlines a method to assess viability of 3D cell models created with RASTRUM™ using fluorescent dyes for distinguishing live and dead cells via fluorescence microscopy. The esterase substrate calcein AM stains live cells green, while the membrane-impermeable DNA dye Ethidium homodimer III (EthD-III) stains dead cells red. Hoechst 33342 nucleic acid stain is cell-permeable and emits blue fluorescence when bound to dsDNA in live and dead cells.