How to check from where FWUpdate will check for updates? I bought a Samsung Note 3 from a non-carrier, and I am using it with a SIM from one of the cheap national carriers. I would like to know from where FWUpdate is checking and downloading updates. Is it getting the information from the carrier? From the default Samsung repo? Possibly from a repo as configured by the store that sold it? How can one check from which repository is the Note checking for and downloading updates?

Note that my goal is not "How can I get the update faster", but rather I'm actually trying to prevent the system from finding and installing the update. After installing Dcentral 1 I see that FWUpdate is trying to update itself at least once per day.
There is no pre-configured repository for the phone to check.

In order to avoid their servers being over-flooded with download requests, Google along with manufacturers and carriers employ what is called a "staged roll-out."  Once they initiate the roll-out for a given region, a specially crafted message that contains the actual download location of the firmware image is sent to the eligible phones.

The carrier, manufacturer, and Google can all initiate such messages. It depends on where you originally purchased the phone who will be responsible for the updates.  This is driven by the phone's model number.  Carrier-branded phones will most likely get the notifications from the carrier, otherwise the manufacturer will handle the upgrade roll-out.  Upgrades for Nexus and Google Play edition lines are handled by Google themselves, usually without manufacturer or carrier intervention (with a rare exception of Verizon Galaxy Nexus model.)

Technically, it's possible to intercept such messages and extract the actual URI of the firmware file for manual download and installation, but you will be dependent on others to do that, if your device has not yet been made eligible for the upgrade.

After a while, when the initial rush of upgrades is over, the manufacturer will usually host the firmware images on their own sites for anyone to download.  It can take a while though.