Consider if the item or person fits into the time period spoken about to identify anachronisms. A character using a cellphone in a Shakespearean play is an anachronism.
Pro Tip 2/3
Cultural References
Anachronisms can be intentional for humor or effect in modern retellings of historical stories. Adding a spaceship to a medieval setting is a deliberate anachronism for comedic effect.
Pro Tip 3/3
Implicit Criticism
Using 'anachronism' may subtly suggest that something is outdated and should be replaced. Calling a school's old-fashioned rules anachronisms implies they need updating.