Final answer:
A judge or magistrate determines the existence of probable cause during an arrest or before issuing a search warrant, as outlined by the Fourth Amendment, which prohibits unreasonable searches and seizures. Probable cause is a threshold that law enforcement officers must meet, albeit lower than that required for a conviction.
Step-by-step explanation:
When an individual is arrested, whether with or without a warrant, the existence of probable cause is ultimately determined by a judge or magistrate. The Fourth Amendment of the U.S. Constitution prohibits unreasonable searches and seizures and necessitates that government officials must demonstrate to a neutral judge or magistrate that probable cause exists before obtaining a search warrant, except in certain circumstances such as consent, exigent circumstances, or when items are in plain view.
In instances where law enforcement officers do not need a warrant, they must still meet the standard of probable cause to justify the arrest, search, or seizure. This legal standard requires less evidence than what is needed for a conviction but still necessitates reasonable grounds to believe a crime has been committed or that evidence will be found. During preliminary hearings, a judge will also make decisions on the admissibility of evidence obtained.