Can the police get a warrant if someone told them I was doing something illegal?
Unfortunately, many warrants are based solely on an informant telling the police that another person committed a crime. Warrants only have to be based on probable cause, which is a much lower standard than the “beyond a reasonable doubt” required for a conviction. This means that the standard for getting a warrant and arresting someone is much lower. As long as the person named in the warrant is a credible person, the warrant can stand on their statements alone. The issue then generally boils down to whether or not that person is credible.
Credibility - Identification
Credibility is usually guided by case law, rather than a specific statute. Appellate courts will tell trial courts what they think are good guidelines for deciding when a person is credible. Most courts do not have a clear test or specific formula for deciding credibility, but instead rely on factors such as "identification." If the person is identifiable, it weighs in favor of credibility. If the person just called in anonymously and refuses to identify themselves to law enforcement, they may not be found credible, and the warrant probably cannot be supported on their testimony alone. The agency will have to identify other evidence in the affidavit.
Credibility - History
The second factor is history. Courts will look to see if this person has a history of being honest or telling the truth. The court will consider whether this person has given reliable information in the past and whether information previously provided has turned out to be true. Many drug cases are based on confidential informants or “snitches.” Even though they may have a criminal history, if the officer conducting the investigation can affirm that the informant or snitch has provided reliable information in the past, they can still be considered a “credible person” for purposes of an arrest warrant.
Some of the most credible informants however, are "citizen informants," just ordinary bystanders who happened to witness a crime or actions that are highly suspicious of criminal activity, such as a neighbor who reported noticing that "stoned-looking" people come and go at all hours and in high volume to and from your home. Many people hope that officers will review information provided by informants and compare that information with physical evidence, if any. Unfortunately, many states require little or no verification of facts for a warrant when someone accuses you of doing something illegal.
If you are the subject of a warrant, you should contact a criminal attorney as soon as possible so that you can contest the warrant (especially if the informant is not credible) and contest any allegations. Failure to contest issues like bad warrants can result in you waiving valuable constitutional rights and protections.