We routinely use AO/PI staining:

"Acridine orange (AO) and propidium iodide (PI) are nuclear staining (nucleic acid binding) dyes. AO is permeable to both live and dead cells and stains all nucleated cells to generate green fluorescence. PI enters dead cells with compromised membranes and stains all dead nucleated cells to generate red fluorescence.  Cells stained with both AO and PI fluoresce red due to Förster resonance energy transfer (FRET), so all live nucleated cells fluoresce green and all dead nucleated cells fluoresce red."

from: http://www.nexcelom.com/Nexcelom-Blog/aopi-viability/

There are several other methods like LDH release assay or ATP assay (maybe not suitable for your problem)..

We routinely use AO/PI staining:

"Acridine orange (AO) and propidium iodide (PI) are nuclear staining (nucleic acid binding) dyes. AO is permeable to both live and dead cells and stains all nucleated cells to generate green fluorescence. PI enters dead cells with compromised membranes and stains all dead nucleated cells to generate red fluorescence.  Cells stained with both AO and PI fluoresce red due to Förster resonance energy transfer (FRET), so all live nucleated cells fluoresce green and all dead nucleated cells fluoresce red."

from: http://www.nexcelom.com/Nexcelom-Blog/aopi-viability/

There are several other methods like LDH release assay or ATP assay (maybe not suitable for your problem)..

AO/PI staining, LDH assays, caspase-3 activity assays, ATP assays, TUNEL... Even cell cycle analysis by flow cytometry may give you information about cell viability if you look at subG1 population.

Neutral red assay, very cheap and easy to perform.

http://www.biovision.com/cell-proliferation-viability-cytotoxicity-810/

https://www.promega.in/resources/product-guides-and-selectors/protocols-and-applications-guide/cell-viability/

thank you very much for your suggestion..I will look up more into those suggested method..thanks again

I prefer PI staining. 0.5/1ug/ml. By FACS. Don't incubate too long.