Hey Kevin, I'm not a reporting master by any means, that would be our Remote Consulting and Professional Services teams. However, when I look at the API explorer
https://experience.workfront.com/s/api-explorer , I can see that the Task object can reference collections for Documents. Documents can have actions ran against them to show them stuff like the getProofURL. I believe if you're good enough, you should be able to use EXISTS or figure out a way to tie the getProofURL action to show if a task has a proof URL or not (thus giving it a value.)
I suspect you could build this as a calculated field, and then set up the display logic on the calculated field, but you would need to make sure that all tasks with documents that are proofed have that calculated field on it, or your data might be inaccurate. Good luck! Dustin Martin Assigned Support Engineer Workfront