WEMA technology typically uses in-tank sensors, so I'd say that's 90% of the solution
As to which tank... can't say, since our boats are different. I can tell you ours is in the fill (port-side) tank, but it would work in either. If you can see the tops of your tanks, even from a distance, or with a mirror or a digital camera... you would likely be able to identify the tank that has the "extra thing" on top.
(Our port tank has the gauge, and the fill on top. Starboard tank has nothing on top. There's a connecting hose that runs from the bottom of each thank through the bilge to level water in both tanks. BTW, in another conversation, if you have a similar cross-over hose, putting a T in that would give you a nifty way to drain tanks if you ever have winterize, or just flush...)
Once you can find the guage, you might be able to get some idea on how to access. In your boat, it might be very easy. In ours, the sendor was simply too long to remove (and the new on was too long to insert) withouth making a way for more straightwarward (mostly straight up and down) access. It happened our access would be via the bottom of a hidden storage area behind a dinette seat, so creating an access hole wasn't a cosmetic issue at all.