A direct measurement of lattice spacing by X-ray diffraction is performed on a periodic array of unit structures provided on a substrate including semiconductor devices. Each unit structure includes a single crystalline strained material region and at least one stress-generating material region. For example, the single crystalline strained material region may be a structure simulating a channel of a field effect transistor, and the at least one stress-generating material region may be a single crystalline semiconductor region in epitaxial alignment with the single crystalline strained material region. The direct measurement can be performed in-situ at various processing states to provide in-line monitoring of the strain in field effect transistors in actual semiconductor devices.