Ludanto, see this thread: http://www.burningwheel.org/forum/showthread.php?13219-Using-wises-to-make-declarations
The rules don’t provide much detail on using wises to establish facts, unfortunately. It’s a bit of a gap, and I find this puzzling as well. (It’s more or less the same in Mouse Guard, by the way – there’s one sentence in the overview chapter telling you that you can use wises to do this, but apart from that the rules are silent on that point.)