The positions of hydraulic fractures may be detected during multistage reservoir stimulation operations. Fracturing fluid is injected into a well at a pressure above the fracturing pressure to produce at least one hydraulic fracture. Then, a marker slug is injected into the well. Next, additional fracturing fluid is injected into the well. When the marker slug enters at least one of the hydraulic fractures, a detectable pressure response is observed, and the position of a hydraulic fracture may be detected from the volume of fracturing fluid injected after injection of the marker slug. The marker slug is a fluid that has a viscosity and/or density that is different from the fracturing fluids injected before and after the marker slug. This technique may be combined with other well operations, such as plugging at least one hydraulic fracture or creating at least one new hydraulic fracture.