I did a bit of research regarding this area. This information is from Criminal Law Handbook: Know Your Rights, Survive the System, 9th edition. My own commentary is in blue.
"If a police officer knocks on your door and asks to enter you should ask to see a search (or arrest) warrant. The officer may have no right to enter your home without a warrant. If the officer displays one (and it has your address on it), allow him to enter. While the officer is inside, observe the officer’s activities and if possible make notes about them. (having cameras in your house would be great if this happened) The notes can help you testify fully and accurately in the event that you later want to challenge the officer’s actions in court.
If the police officer does not have a warrant you may decide to allow the officer to enter your dwelling anyway. You will have “consented” to the entry and you will probably have no right to challenge the search later in court. (Don’t do this because you don’t know what the Cop has on you)
If the officer doesn’t have a warrant, you can tell the officer that you refuse entry into your dwelling. (Use a tactic mentioned in the beginning of the book – ask the officer if he can come back at a later time. That way you can contact a lawyer. Also ask if a valid search has been issued) You may do so loudly enough for others to hear, so that they may testify to your refusal in court if necessary. The officer may insist on entering anyway, and if so you should not try to interfere. Here’s why: 1) It’s much safer to challenge a police’s actions in court than in your home. Also, you don’t want to be charged with interfering with an officer. 2) Perhaps a valid warrant has been issued, even though the officer doesn’t have it. If so, the officer probably has the right to enter your dwelling. 3) The officer may have a legal right to enter your dwelling without a warrant. Police officers often have the right to conduct searches and make arrests without a warrant ( the smell of marijuana is one thing that would allow them to make a search)
If the officer insists on entering your dwelling despite your refusal and the absence of a warrant, it is even more important that you observe and make a record of the officer’s activities that you can refer to should you challenge the officer’s actions in court."