In most states, an officer can't arrest an adult for a misdemeanor without a warrant, unless he witnesses the person committing it. In other words, the misdemeanor must occur "in the presence" of the officer. But what does this requirement really mean?
In general, for an officer to make a warrantless arrest for a misdemeanor, courts require that the officer have probable cause to believe that someone has committed a misdemeanor in his presence. As long as the officer has probable cause, the arrest is valid even if the suspect didn't actually commit a crime or is never convicted. (Atwater v. Lago Vista, 532 U.S. 318 (2001).)
States have different standards when it comes to the requirement that a misdemeanor occur in an officer's presence. Here are some general guidelines on what officers can and can't rely on to form probable cause for a warrantless misdemeanor arrest.
An officer can use any of her senses to witness the crime, including hearing and smell. For example, if an officer smells marijuana on someone, she may have probable cause to make an arrest for possession of marijuana. (Rife v. D.T. Corner, Inc., 641 N.W.2d 761 (Iowa 2002).)
An officer can rely on scientific instruments to help her witness an offense, such as binoculars, electric amplification, or radar. For example, an officer can use a radar gun to determine a driver's speed. (Carranza v. State, 266 Ga. 263, 467 S.E.2d 315 (1996).)
Depending on the state's law, an officer can use common sense and reasonable inferences to find probable cause that a misdemeanor has occurred. For example, suppose an officer doesn't witness a DUI accident, but arrives on the scene and sees a person sitting in the driver's seat taking off his seat belt. The officer may be able to infer that this person was the driver and make a warrantless arrest if the other evidence suggests it was a DUI collision. (State v. Harker, 240 P.3d 780 (Utah 2010).)
Likewise, an officer can use cues such as his own speed to determine probable cause that a driver is speeding. (U.S. v. Miller, 589 F.2d 1117 (1st Cir.1978).)
If an officer didn't witness a misdemeanor, she cannot base an arrest on what other people tell her happened. For example, if a security guard tells the officer about a store theft he witnessed, or the officer's only information comes from security videotapes, the offense didn't happen "in the presence" of the officer. (Forgie-Buccioni v. Hannaford Bros., Inc., 413 F.3d 175 (1st Cir. 2005).)
The story is different, however, if a citizen makes his or her own arrest. In many states, private citizens have the authority to make lawful arrests. A citizen's arrest takes place when a citizen detains—or directs law enforcement to detain—someone. The law on citizens' arrests differs from state to state. But a citizen's arrest where the citizen provides an officer with the information to make a warrantless arrest is an important exception to the "in the presence" requirement for misdemeanors. (For more on citizens' arrests, including the danger of citizens actually carrying them out, see Legal Trouble from Citizens' Arrests.)
One officer can rely on the statements of another. If an officer observes someone commit a misdemeanor and relays that information to another officer, the second officer can typically make an arrest without a warrant. (Brown v. State, 442 N.E.2d 1109 (Ind. 1982).)
An officer can also rely on a suspect's admission that he is committing or just committed an offense: Even if the officer wouldn't have otherwise known about the misdemeanor, he can usually carry out an arrest. (State v. Gulczynski, 32 Del. 120, 120 A. 88 (1922).)
If a misdemeanor doesn't occur in the officer's presence, he can try to get a warrant for the suspect's arrest. The officer submits a written statement under oath (an "affidavit") to a judge that sets forth what he knows or heard from witnesses. If the judge determines the affidavit establishes probable cause that the suspect committed a misdemeanor, she will sign the arrest warrant.
If you are arrested for any reason, be sure to consult to a criminal defense attorney about the circumstances of your arrest (and your case). The law may differ from one state to another—for instance, on the issue of delay between an officer observing a misdemeanor and making an arrest. If there's a question as to what the arresting officer did or didn't witness, you may be able to challenge the legality of your arrest.