When police search a person or their property, there are guidelines that must be followed in order to meet requirements under the law. This is because a person is entitled to a certain level of privacy under the Constitution. However, if a person is breaking the law, police officers have rights in certain instances to step in and intervene.
The 4th Amendment is in place to protect citizens from illegal searches and seizures. So, how do you know if the search was reasonable? Either a judge issues a search warrant based on probable cause, or a certain situation can arise that justifies a search without a warrant. For those searches that were performed in the moment, what justifies a reasonable search?
Certain circumstances must exist to justify a search when a search warrant hasn't been issued. Whether that be erratic driving, the presence or suspicion of a weapon or some other incident that directly precedes the search, these factors can come into play. When a suspect is accused of a drug crime, officers can allege that they found drugs, drug paraphernalia or some related content on a person or on their property. Property could be a vehicle or a home in these situations; it must be something owned by that person that they are responsible for.
It's also important to understand that the 4th Amendment doesn't apply to every situation. The 4th Amendment becomes irrelevant if a person is in a situation in which a legitimate expectation of privacy doesn't exist. Each person's search will be different and will be affected by different factors.
Source: FindLaw, "Search Warrant Requirements," Accessed May 14, 2018