How do you know if a cell culture is contaminated?

Cell culture can be contaminated in a number of ways, broadly classified as chemical contamination (endotoxins, free radicals, heavy metals, plasticizers, detergents or disinfectants) and biological contamination (Bacteria, Yeast, Mold, Viruses, Mycoplasmas, Protozoa, Invertebrates and cross-contamination from other cell lines). The contamination can be detected by visual inspection (e.g. detecting cloudiness in the media); microscopic examination (which can detect bacteria and viruses); testing pH (as contaminants may increase or decrease the pH).