for non- professional researchers, simple and direct in vitro assay using for primary testing of new compounds before application of another specific assays in vitro and in vivo.
In cell cultures, the MTT assay is commonly used (doi: 10.1016/0022-1759(83)90303-4). Alternatively, fluorescein diacetat can be used (doi: 10.3109/10520297209116483).
Just to add few thought, Cell viability/proliferation is the easiest thing you could asses to screen a library of compounds. I personally prefer XTT assay over MTT assay. It is quicker and simpler. You will have to grow the cells in a 96 well plate and treat them for 72-96 hours with your drug and then read the plates after adding XTT.