Hi
@Cherith,
I don't know of any Apple Watch apps that allows you to scan a sensor (there's also the practical problem of scanning if your sensor if its on the same arm as your watch).
There are various option for getting your BG data onto your watch - the specifics will depend on your set up (esp Apple vs Android).
Other thing to be aware of - Apple limits how often it refreshes data displayed on 'complications' (to conserve battery life). The exception to this is calendar info which refreshes in realtime. As a result some complications rely on you setting up a dedicated calendar and then subscribing to it. The main thing to be aware of is data security (your BG data will be open)... not all apps work that way, and just accept the data displayed is a little stale.