A molecule demonstrates resonance when it possesses delocalized electrons within a conjugated system characterized by alternating single and double bonds.

Resonance is a fundamental concept in chemistry that refers to the delocalization of electrons across certain molecules. This phenomenon arises in molecules where the true structure cannot be accurately represented by a single Lewis structure. Instead, these molecules exist as a hybrid of multiple configurations, called resonance structures.

To ascertain whether a molecule exhibits resonance, one should identify a conjugated system. A conjugated system involves a scenario where a single electron occupies a $p$ orbital and is adjacent to a double bond. This arrangement facilitates the delocalization of electrons, which is a defining characteristic of resonance.

A classic example of a molecule exhibiting resonance is benzene. Benzene features a six-membered carbon ring with alternating single and double bonds. The position of the double bonds can shift around the ring, generating various resonance structures. However, the actual structure of benzene is a hybrid of these forms, with electrons delocalized throughout the ring.

Another method to identify resonance is by examining molecules that exhibit a charge distributed across two or more atoms. For instance, the nitrate ion ($\text{NO}_3^-$) has a negative charge that is spread among its three oxygen atoms, indicating the presence of resonance.

In summary, to determine if a molecule exhibits resonance, look for a conjugated system of alternating single and double bonds or for a charge distributed across two or more atoms. These features signify the delocalization of electrons, which is a critical aspect of resonance.