Many people think that if they have been searched and there is no warrant -- the results of the search are going to get thrown out. Maybe, but there are a number of exceptions to the requirement that officers must have a search warrant before they search. These exceptions are set forth below.
Search Incident to Arrest
If the officers make a lawful arrest then they may search the suspect. The search extends to the full body (but no trip searches in the field generally) and the immediate area around it where an arrestee might have possession of a weapon. Persons in the vicinity may also be searched if the officers have a reason to believe they may present a danger or destroy evidence. The exception to the exception is protective sweeps which prevents a search of the house where the arrestee was located unless there is reason to believe that search is necessary to protect officers or others.
A fairly obvious exception. If an officer stops you in the car and asks if he can look around, you have consented to the search and waived your 4th amendment rights.
Your consent must be free and voluntary, without threat of force. Note that a warrantless search may be justified by consent even if a 3rd party who has common authority over the property consents. Examples: a landlord to common areas of the building; a tenant against a landlord, an employer with regard to an employee's desk, a husband or wife against each other, a co-occupant against another, a person to whom you lend your car against you.
If you are arrested and booked, a second search may occur at your booking to protect your property, to protect the officer against unwarranted claims of lost or stolen property and to protect the police from potential dangers. This rule also applies to impounded motor vehicles. This may include closed containers that would ordinarily not be subject to search if their contents cannot be ascertained.
This large exception eliminates the need for some of the lessor exceptions already discussed. Because an automobile is mobile, and it is driven on a public thoroughfare with occupants and contents available to public view, an exception has been created for vehicles in transit known as "fleeting targets." There must be probable cause to believe to believe the car contains evidence or contraband. The limitations apply to a house under surveillance where the car is in the garage and there is time to get a warrant or a car mounted on blocks not capable of movement. A motorhome, however, even if used as a residence, may be searched without a warrant.
This exception allows officers to search based on probable cause and exigent circumstances which create a compelling need for immediate action due to danger to life or public safety. Exigent circumstances may also justify a search when officers are in hot pursuit of a suspect and there is reasonable cause to arrest.
Seems obvious but officers may search objects in plain view only when they have probable cause to believe that the item in question is evidence of a crime, contraband, or otherwise subject to seizure. And the baseline requirement is that the officer must be in a place where he has a right to be when he observes the object to be seized. Subdivisions of this exception are plain touch and plain smell.
An administrative search differs in nature and purpose from criminal searches. These searches are justified by whether a public interest justifies the intrusion contemplated. These searches generally apply to questions of fire codes, health, safety and housing codes, licensing provisions. The object is not to arrest the person whose property is searched (although that could happen) but the protection of the public.
You probably can't. That's one reason to hire a good lawyer if you are arrested so he can explore the legality of any searches conducted. If the search is illegal the evidence is suppressed no matter how incriminating.
The mistake made by most people under suspicion is that they think by cooperation they can avoid arrest. NOT! Never make a statement to the police except in the presence of a lawyer and never never consent to a search. You don't have to and the police can't get angry if you refuse. No talk, no search. In the long run you'll be glad you didn't.