How to know the last owner of an object?

How to know the last owner of an object? Is there a script for it? Or just a function? Or other process available for it? ... ?

You can use the changed(integer change) event:

    changed (integer change)
    {
        
        if (change & CHANGED_OWNER)
        {
            llSay (0, "New owner key is: " + llGetOwner() );
        }
    }

by storing it in the script, most likely in a list.

key oldowner;//memorizes llGetOwner() before it can change.

on_rez{
oldowner=llGetOwner(); //memorize the actual owner of the prim. it is the old owner on the first rez IF the prim has been copied to someone elses inventory.
llResetScript;//sets llGetOwner() to actual prim owner, if the prim owner changed since the last rez llGetOwner() will now return aa different key.
}state_entry{
if (oldowner!=llGetOwner());llSetObjectDesc( llKey2Name (oldowner)); //if the owner changed, store NAME of old owner in object description the first time the object is rezzed by a changed owner.
oldowner=llGetOwner();//memorize actual prim owner in case the owner will change inworld after rezzing.
}changed (integer change){
if (change & CHANGED_OWNER){
llSetObjectDesc( llKey2Name (oldowner));//stores NAME of the old owner in object description.
}}

does state entry fire before on rez ? I honestly can't remember... might want to have the above code from state entry in a function called by both on rez and state entry if it doesnt

Unfortunately there's no way to tell FOR SURE. If a resident obtains an object, then passes it along without having attached/detached it, or rezzed in-world and re-taken it, there's no way for a script to determine that person ever owned it. My suggestion is to remove from your design/requirements any dependence on having to know the ownership history of the object. Or start caring only about who has (owned AND) USED the object maybe....

Yes. There's no way to stuff into, or start a script in, an object that's not rezzed, so if the script is running at the time you rez the object, the script will definitely already have had its default state_entry called.

Yes, that's really my concern. For example, resident A sent an object to resident B by draging it from the inventory of A. How does the object know its last owner is A?

Answer: it can't.