The standard heat of formation (ΔH°) of a compound is described as the enthalpy change when the compound is formed from its separated elements.  When the heat of formation for a compound is different from either a prediction or a reference compound, this difference can often be attributed to strain.  For example, ΔH° for cyclohexane is -29.9 kcal mol while ΔH° for methylcyclopentane is -25.5 kcal mol.  Despite having the same atoms and number of bonds, methylcyclopentane is higher in energy than cyclohexane.  This difference in energy can be attributed to the ring strain of a five-membered ring which is absent in cyclohexane. Experimentally, strain energy is often determined using heats of combustion which is typically an easy experiment to perform.
Determining the strain energy within a molecule requires knowledge of the expected internal energy without the strain.  There are two ways do this.  First, one could compare to a similar compound that lacks strain, such as in the previous methylcyclohexane example.  Unfortunately, it can often be difficult to obtain a suitable compound.  An alternative is to use Benson group increment theory.  As long as suitable group increments are available for the atoms within a compound, a prediction of ΔH° can be made.  If the experimental ΔH° differs from the predicted ΔH°, this difference in energy can be attributed to strain energy.