The best way of detecting the presence of viruses in the infected cell culture is:

The changes in cell morphology caused by infecting virus are called cytopathic effects (CPE). In order to view these cellular changes under light microscopy, the cell cultures have been stained with hematoxylin, a basic dye, and eosin, an acidic dye.  Common examples are rounding of the infected cell, fusion with adjacent cells to form a syncytia (polykaryocytes), and the appearance of nuclear or cytoplasmic inclusion bodies.