Well, if we hadn't had an update saying that Isaac's also a Lookamancer, then I'd be very conflicted! It looks like lookamancy because, well, it looks like he's Looking at something with the way the spell looks, but Isaac's a thinkamancer... but as it is, we do have other source of information - from the extra book pages, we know that Isaac's also a master-class Lookamancer!
So you're right that Wanda casting a spell isn't much evidence for a discipline, because it's been stated that she's proficient with lots of disciplines and it hasn't been stated which ones.
...you know, on second thought, I think I'd be more general - I trust *narrative* reasons much more than anything else. If the text makes it superficially obvious that a spell should come from a particular discipline - using whatever narrative methods the comic feels like using, from sound effects to visual effects to who casts it to what people say about it - I trust that, and maybe use that to try to figure out generalities, like what the disciplines mean or what spells should go in which discipline. It's going to be a long time before I ever accept any reasoning of the form "well, X spell seemed superficially like this discipline, but because of its effects it probably isn't."
Just read http://www.erfworld.com/wiki/index.php/LIAB_Text_30
. Ace Hardware talks about how accessories are his favorite Dollamancy, so much better than those stupid cloth golems. Then later, he makes accessories - a jetpack, a laser pistol, something else. It seems pretty clear based on the text that all those things are dollamancy.