Fluorescence-Activated Cell Sorter (FACS) or flow cytometry has been widely used for cell cycle studies.

A suspension of dead or live cells is passed single-file through a laser beam by continuous flow of a fine stream of the suspension. Each cell scatters some of the laser light, and also emits fluorescent light excited by the laser. Fluorescence intensities are typically measured at several different wavelengths simultaneously for each cell, allowing users to mark cells with different fluorescent probes. For example, cells can be marked with a fluorescent probe for DNA content (G1/S or G2/M phase) and simultaneously measured for incorporation of BrdU (S phase).

A good starting point for FACS resources, tips, and protocols is Scripps FACS Core Facility.