So Fiona and her mages, who had no prior military training and lived in seclusion for most of their adult lives reading books and studying, could scout in a similar fashion to Leliana's people?
If they can't, then they are militarily inept and this is another indicator that they shouldn't have been so reckless as to pick a military confrontation they weren't prepared or equipped to fight.
Of course, the mages themselves don't have to be the scouts- they could, especially if they had useful/relevant magic, but they could just hire or recruit scouts on their behalf. Just as they could have arranged spy networks, built alliances in advance, prepare fortifications or escape routes, and so on. There was even an entire fraternity based around accumulating wealth, which would have been well placed to buy and leverage the loyalty and services of mundanes. Scouts and spies are one of the easier things to get started, within a point, and could have been started well before the independence kickoff.
If you're going to purposely start an open revolt, and you are incapable of conducting basic military operations and capabilities because of inexperience, then 'inept' is a wonderful description of your revolutionary competence.