From my understanding, one of the most practiced procedures to identify necrosis (being a difficult occurrence to detect on its own) is to first differentiate if it's necrosis or apoptosis.

You may ask your self what Apoptosis is.

Apoptosis:
A natural process of self-destruction in certain cells, such as epithelial cells and erythrocytes, that are genetically programmed to have a limited life span or are damaged. Apoptosis can be induced either by a stimulus, such as irradiation or toxic drugs, or by removal of a repressor agent. The cells disintegrate into membrane-bound particles that are then eliminated by phagocytosis. Also called programmed cell death.

Being apoptosis a very common phenomenon during embryogenesis as well as adult life, and a frequent response of cells to drugs in the course of therapy, its quantitative evaluation represents an issue of considerable relevance. Flow cytometry is the choice technique for the quantization of apoptosis; this will allow the isolation of whether Necrosis is even existent in the subject tissue.

although it would seem apparent that this answer is deviating from the actual question. It's important to notice that according to people in the medical field, quantative measurement of necrosis can be a difficult task with out first identifying where or not necrosis is due to Apoptosis.

Flow cytometry:
A technique for identifying and sorting cells and their components (as DNA) by staining with a fluorescent dye and detecting the fluorescence usually by laser beam illumination.

But to answer your question more directly, I found a journal that use the following method to quantify affected tissue:
..." Individual wells of infected and control cultures were stained with PI and AV (Roche) according to the manufacturer's methods. Slides were washed and fixed with 10% buffered formulin, and cells were viewed at a magnification of ×400 and were counted and photographed using a Zeiss fluorescent microscope (488-nm excitation and 515-nm long pass filter). " (3rd citation)

hope this helps.....