Quickest technique I've used is Trypan blue exclusion but fluorescein diacetate is cheap, quick and will provide indication of plasma membrane integrity. The dye is converted by cytoplasmic esterases and fluorescent product remains trapped in cytoplasm of viable cells. The commercial live/dead assay stain exploits similar principle but includes a nuclear dye for dead cells.
Sytox green is DNA binding flouorescent dye and taken up the necrotic cells is also a good option. analysis is possible with fluorescent microscope. for further detalis you may check the product;