List a few ways that would lead you to believe a chemical reaction has occurred

There are many ways which can lead us to believe a chemical reaction has occurred. I am discussing few of them.

Change in Color:
                          Few reactions proceed with the change in colour. When Bromine Water (yellow-Brown) is treated with unsaturated compounds, the color disappear as the reaction is completed.

Gas Production:
                         Some reactions proceed with the elimination of gas i.e. CO₂ , H₂, N₂, H₂O e.t.c. 

Temperature Change:
                                 Some reactions evolve or absorb heat when proceed. This change is the evidence of progress of reaction.

Thin Layer Chromatography:
                                             Very important technique in detecting the progress of reaction by determining the number of spots for UV active compounds on TLC plate. Even non UV active compounds can be detected by using other locating agents.

Instrumental Techniques:
                                       Modern techniques like IR, MS, NMR, UV spectroscopy can be used to detect the progress of reaction.

By using any one technique like boiling, distillation, condensation ,etc , one can be used to distinguish between a pure substance and a homogeneous mixture.

Explanation:

If the liquid is a pure substance, then it can not be separated by using physical means because all atoms of a pure substance show same properties.

But on the other hand, if there is homogeneous mixture, it can be separated by using techniques like boiling, distillation, condensation ,etc.

Thus, using any of the one method listed above can be used to distinguish between a pure substance and a homogeneous mixture.