You can put the portal in a specific plot linked to a beacon and see the type of traffic income you get. That isn’t a great way but at least something some people do to decide traffic… I would assume, though, with the amount of portals that are available in the game currently because of how easy it is to farm Oort that traffic throughput is down for at least some people…
I did a pretty extensive post on portal traffic when I was running one of the 2 portal networks in the game. You can see the post below. I would assume in some cases most of the findings are still relevant.