The likelihood of an element being reduced in a reaction depends on its position in the electrochemical series, also known as the reactivity series. Reduction involves the gain of electrons, and elements that readily accept electrons are more likely to be reduced.
In general, elements that are more electronegative (like halogens such as fluorine and chlorine) or those that have a high standard reduction potential (measured in volts) are likely to be reduced. For example, in a reaction involving metals and non-metals, non-metals are typically reduced.
To assess specific scenarios, you would look at the standard reduction potentials for the elements involved. The element with the highest standard reduction potential in the reaction will be the one most likely to be reduced. If you have a particular reaction in mind, I can help you analyze it further!