Note: I represent Optimal Workshop, the company who makes the tool I propose below.
I suggest you try Reverse Card Sorting, also known as Tree Testing. This is done once you have formed your proposed/prototypical IA. An online Tree Testing tool, such as Treejack, helps you validate your IA (and the labels you want to use in it) in a task based context. This is instead of a series of Closed Card sorts. You may find it beneficial to do multiple Tree Tests to refine your IA by comparing the results of various labeling strategies before moving on to other stages of the project (so don't use up your whole database of potential study participants in the first hit). If your tree is large (thousands of content items) it could also be useful to segment your users based on behaviour profiles and to test each segment with appropriate tasks against the same tree (IA).