Parameters and techniques for detecting the location of three-dimensional crossflow separations are evaluated using several data sets. Several definitions of separations and the physics of the separation process are discussed along with descriptions of the separated flowfield. Measurement techniques that depend on each of these descriptions are then considered, and data are compared and contrasted. The data analyzed here represent a very rare combination of many different measurement techniques applied to the same geometry and apparatus from several different studies, including oil flow visualization, laser Doppler velocimetry, surface pressure, and surface hot-film skin-friction measurements (magnitude only and directional). Pressure is the least sensitive of the indicators of separation, although minima in rms pressure fluctuations correlate well with separation location. Hot-film skin-friction magnitude measurement is one of the easiest and most accurate techniques; local minima correlate well with separation location. Laser Doppler velocimeter measurements provide the most detail about the separation flowfield but at great expense and with the limitation of requiring knowledge of the separation line direction