I think they'll just watch the games he's played, actually. When it comes to defending it's mostly about positioning, decision-making and intent. They'll also look at the games he's missed through injury.I'd imagine his suitors are looking at advanced statistics to determine how much value he would bring to their side, how he'd work in a good team defence, small areas of improvement that he could make to fit in, etc.
A curve ball in all of this is that clubs could offer an incentivised contract, which would be good news for Ben (as he might stand to earn 800k as long as he plays more than 15 games per year), but bad news for North because his base salary is all they take into account for FA compensation.