How do I tell which Apple Watch I have to order a new band?

The case size, 38, 40, 42, or 44mm, is listed in the iPhone Watch app, among other places.

iPhone Watch app > My Watch > check the info just below the search box at the top.

You’ll see the name you’re using with the Watch (which can be changed), and the case size.

Select the watch info, if you happen to have more than one Watch paired and you’re looking for “the other” Watch.

Press the Digital Crown on the side and select settings then general and then about. This gives you the version along with capacity,, space available and lots of other goodies.