That seems like an odd message to include in an emergency action system, which by definition is only active in unexpected situations. Is there really no system to confirm if a fuel leak is happening?
Nope, there is no system to confirm a leak apart from a camera around the tail if you're lucky enough to have one, my previous airline had a flight where an engine leak was detected this way. Think about it, how would you design such a system? So this falls on the crew.
The procedure to determine if you have a leak is pretty much the same across types: add the fuel on board (FOB) to the fuel used (FU) and make sure that the number you get is the same as what you started the flight with. If it's less by some margin then you probably have a leak. You can confirm further by looking at tank quantities (but they take time to reduce depending on the size of the hole). If you get an engine or pylon leak then you might also see increased fuel flow on that engine. If the leak is elsewhere in the system then you might notice a smell. If you can't work it out then the procedure (at least on Airbus types) usually involves turning an engine off to see if the leak stops (yep, really).
As for the ECAM "open fuel transfer valves" message, I don't know for sure on the 380 but all the other Airbus types I've flown have something like:
.IF NO FUEL LEAK
FUEL IMBALANCE....MONITOR
So it doesn't really instruct you to open the transfer valves but leads you into the fuel imbalance procedure if you think you need it. The very first line of the fuel imbalance procedure says something like "Don't apply this procedure if fuel leak is suspected".
At its simplest you measure estimated volume delivered to the engines against estimated volume remaining in the tank. Both are things that should be digitally measurable.
The problem seems to be that the only case it really matters is in a catastrophic accident where such measurements are going to be broken anyways.
E.g. the A330 has an inner tank in each wing (which itself can be split into two compartments if damaged), an outer tank in each wing and fuel in the horizontal stabiliser which is used for CG control in the cruise. All of that plumbing can leak too. You’d be adding significant weight and complexity implementing leak detection across all that.
Regardless of all of this, the aircraft is still fully controllable even with a total asymmetry (one side empty the other full) so balancing the tanks isn’t a massive priority.
The engines have predictable fuel consumption patterns. Even if fuel move across a bunch of tanks, you can still calculate total onboard fuels and detect a leak.
The challenge is knowing where the leak is.
Given that the aircraft can be landed over max landing weight (needs a maintenance inspection) and is still controllable with total imbalance I’d say that balancing just wasn’t as pressing of a concern.
Also, with that much damage you never really know where else it could be leaking. Leaking fuel into critical spaces of the aircraft could be bad so turning on the fuel crossfeed might add extra issues.