Each county assessor's office has a computer terminal with an alphabetical index by street name and address of all property in that county.
You use this address index to get the assessor's parcel number for an address, which is a unique identifying number the assessor assigns to each piece of property in a county.
You then can look up the assessor's parcel number to find out who owns the property and its assessed value (note: you'll have to go to the assessor's office in person to find the name of the owner. You cannot find ownership information online).
Sample Address Search
To search an address to get its assessor's parcel number, go to the Alameda County Assessor's Office onilne database for
Type a street address into the search boxes, such as:
2180 Milvia Street, Berkeley
(which is the address for Berkeley City Hall)
At the new page the assessor's parcel number is listed at the top: