It depends on what is being reacted. If, say, you predict a gas will be formed, you might see bubbles. Other evidence of reactions having occurred include color changes, new/different odors (although I wouldn't go sniffing around for any; if you want to check this, you'll have to waft the odor over with your hand), and the formation of precipitates (solids). Specific examples follow:
A solution of diamminesilver(I) chloride is reacted with dilute nitric acid. Silver chloride results as a precipitate, which could be predicted because it's the driving force of the reaction.
Aluminum metal reacts with a solution of copper(II) chloride. There are many ways to tell a reaction has happened (or should have, anyway) for this particular example. For one thing, the metal dissolves while the solution's temperature increases. Also, a red precipitate forms and the solution goes from blue to clear.
Hopefully this has addressed your question! If you have any further questions, feel free to ask!
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