I don't think it is reasonable to expect the FA to notice the leak. They may not be able to see the end of the wing from their seat. Depending on the plane, the pilots can sometimes see the ends of the wings.

I'm not a pilot, but I don't think it is easy to detect a leak. I don't believe fuel gauges are very accurate in general. I believe the pilots (and computers) frequently monitor the burn rate during flight to look for possible issues well ahead of time to ensure they don't run out.

I've always thought fuel leaks were a dime a dozen in aviation (not sure about commercial aviation). Sure, you don't want jp5 all over the place, but it's not going to be like die hard 2 either. Sometimes I wonder if they are more concerned about the environmental effects of a hazmat spill.