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.

Less stable  hope this