This is always a problem, because so many folks who look like they could use an upgrade are probably flying on Tango fares. If you can get a sense from the GA about what the u/g waitlist is like, then you could always strike up a conversation with the person to find out a little bit about them, and maybe see their boarding pass. It's always awkward if you have to tell them that you're going to try to u/g them, and then find it doesn't go through.