text: Water is particularly common solvent to be found in crystals because it is small and polar.  But all solvents can be found in some host crystals.  Water is noteworthy because it is reactive, whereas other solvents such as benzene are considered to be chemically innocuous.  Occasionally more than one solvent is found in a crystal, and often the stoichiometry is variable, reflected in the crystallographic concept of "partial occupancy". It is common and conventional for a chemist to "dry" a sample with a combination of vacuum and heat "to constant weight".
  For other solvents of crystallization, analysis is conveniently accomplished by dissolving the sample in a deuterated solvent and analyzing the sample for solvent signals by NMR spectroscopy. Single crystal X-ray crystallography is often able to detect the presence of these solvents of crystallization as well. Other methods may be currently available.