How to check from where FWUpdate will check for updates?
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.