Your Rights and How to use Them

LAW: You have the right to watch the police when they stop someone.
REALITY: The police get very nervous when people watch their actions. They may threaten you with arrest for 'interfering' (Penal Code section 148). Tell them politely but firmly that you don't want to interfere, but you have a right to observe. Stepback if necessary.

LAW: Police must identify themselves to the public. Officers must wear a nameplate or badge number while on duty (PC 830.10)
REALITY: Those tiny plates are hard to read, and police will often refuse to give you their name and number. Ask insistently until you get it. If you are being ticketed, one of the officers' names will be on the ticket; if you are arrested, most of them should be in the police report which you can get from your lawyer or public defender.

In a casual encounter, the officer has no legal right to talk to you, but is 'inviting' you to talk
A detention is when the police have "reasonable suspicion": specific reasons (not a hunch, not a generalization) to believe that you committed a specific crime.
Finally, the police can arrest you when they have "probable cause" to believe you committed a crime. This is more than reasonable suspicion, but less than complete proof. They don't need to have a warrant already to arrest you.

If you were just a witness to a crime, the police cannot legally detain you.

If an officer starts talking to you, for whatever reason, ask "Am I free to go?" If yes, don't talk, just leave.

If the Officer say no, then you're being detained. Ask "Can you explain why your detaining me?" and memorize the exact words of the officer's response - this could be helpful in court if your arrested. If there are any witnesses, repeat the officer's response "(Oh, so I'm being detained for...?) so they can hear it.

REALITY: Even if the police don't read you your rights, they are in effect. People often talk to the cops because they don't know they don't have to. Although it can be difficult to refuse to answer any questions, it really is the best idea. Tell the police that you don't have anything to hide, but that you've been told not to answer any questions without your lawyer present. Don't try to talk your way out of anything. Even the truth can be manipulated and used against you. If you get confused and start talking, you can stop and assert your right to silence at any time. The police may tempt you by saying that if you answer a certain question, they will let you go, but this is often just a trick.

When you don't answer questions, the police may threaten to arrest you for refusing to cooperate with their investigation. This threat is illegal.

LAW: You don't have to show ID to the police, although you do have to show your drivers license if your pulled over while driving.
REALITY: In a casual encounter, don't even give the police your name - just walk away. If your being detained, you may want to give your name, but nothing more; the police have a right to take you in until you've been identified. If you've been ticketed, you should probably show your ID. If you're being arrested and taken in, you may have to show ID to be released.

If your citizenship is not secure (such as if your an undocumented immigrant), don't even give your name.

LAW: When the police get your ID, they can run a warrant check. This basically checks if you have tickets you haven't paid in a long time, or charges against you which you may not be aware of. If you have a warrant, they will arrest you.
REALITY: The computer system, or the police, may make a 'mistake' and say you have a warrant when you don't. Keep copies at home of all your tickets and of the checks you paid the tickets with.

LAW: If the police arrest you for an infraction (such as jaywalking or possession of less than an ounce of marijuana), they should give you a ticket and let you go. You may get a pat down for weapons, but you should not be searched. If you don't have ID, the police may take you to the station to "verify your identity". If your arrested for a misdemeanor, you may or may not be taken in. If the police arrest you for a felony, you will be taken to the station and booked.
REALITY: Whether you are charged with a misdemeanor or a felony often depends on how the cop feels about you, not on what you've done or not done. It is common for police to 'over-book' people by adding on extra charges like resisting arrest or blocking the sidewalk.