I live in another state and am planning on visiting cali sometime this summer. I have done my research on calif. laws and from what I have determined is that I can not bring my AR-15 in the state for any kind of shooting purposes other then competition.

So my questions are as follows:

When you take your "evil" assault weapon to a public range is there some kind of card you must have on your posession to show that your assault weapon is registered? Say for example I went to a public shooting range and wanted to shoot an AR-15, would I get harrassed to prove that my rifle was registered? Would I need to show proof to the range operators that the rifle that I was useing is a registered rifle that complies with california's registering of assualt weapons?
second question:

Say you are out like on a houseboat on Lake Shasta, you pull up to a cove and do some target shooting, someone sees you,or hears you,
reports it, and the cops show up. At that point in time do you have to show proof that your  AR-15, or any other assualt weapon is registered, or do they just confiscate your weapon until you can prove that it is registered? How is this handled by the police?

I am just wondering how does anybody else other then the government know that your assualt weapon is registered, do you have to carry around proof.

Any help on these questions would be greatly appreciated!!!

If you had registered your assault weapon before the deadline, then you are now on a database that has your name and the serial number of your gun. No, you are NOT required to carry around proof that your assault weapon is registered. If a police officer wants to verify the legality of your gun, he can do it on the spot by checking the database to see if that gun is on there. If it isn't, you are screwed. You do receive a certificate from the DOJ stating your gun was registered, but because of that database you don't ever have to keep it with you. So lets say the cops do show up, they can verify it right on the spot. if it's not on the list, i think its a felony for being in possession of a non registered assault weapon, and you get cuffed and off to jail. So don't risk it, its really not worth it.

Geez, first MG's, now this.  "Papers Please!"

AFIK, it would have to be an official event, and a detour such as on a houseboat would get you a felony.
In other words, you go to the shooting competition, shoot, and leave. I don't believe that your type of plans would fly. I don't think there is a card, but I'd sure carry proof of registration or that I was signed up in a shooting event. The environment in CA is such that you'd be taking some chances. Depending on the agency and the officer, I'd say anything could happen. I've heard stories (make that read, as in newspaper) about cops confiscating perfectly legal handguns and when the owners give up trying to get them back, they give them to judges and da's for brownie points. When even cops have a hard time trying to buy an AW here, well, you can do the math. You could buy a FAB 10 Reciever I've been hearing about.
And I'd only shoot where you have express permission from a property owner or at a range.