How do we know whether a redox reaction is taking place in an acidic or alkaline medium?

Redox reactions are those that involve both oxidation and reduction. In the oxidation half reaction, one or more electrons are lost, while in the reduction half reaction, one or more electrons are gained. When these reactions are combined, we end up with a redox reaction. These reactions can take place in either acidic or basic or neutral medium. If the redox reaction takes place in an acidic medium, proton (or ) will appear in the chemical equation for redox reaction. On the other hand, if the reaction took place in an alkaline medium, hydroxyl ion (or ) ion will appear in either sides of the redox reaction equation. If neither of these ions are seen in the redox reaction, the medium is neutral.

The presence of these ions is used to balance the redox reaction equation. 

Hope this helps.