It depends what biological molecules and what sort of structural changes. Unwinding of DNA is detectable by an increase in UV absorbance. Secondary structural changes in proteins can be detected as changes in the far-UV circular dichroism (CD) spectrum, which requires a CD spectrometer. Tertiary structure changes can sometimes be detected as changes in the near-UV CD spectrum, or in the intrinsic fluorescence spectrum using a spectrofluorometer. Fourier transform infrared spectroscopy (FT-IR) can also be used to investigate aspects of protein secondary and tertiary structure.

However, to get highly detailed information about structural changes, the best methods are multidimensional NMR and x-ray crystallography. Recent advances in cryo-electron microscopy are also making it possible to get detailed structural information about macromolecules by this method.

Sorry, that's not my subject.