on modern airplane the features are very helpful .
fmc equiped airplane help a lot in recognising a fuel leak as some messages :
"USING RSV FUEL"
"INSUFFICIENT FUEL "
"CHECK FMC FUEL QUANTITY "
so we can have
-fuel IMBAL indication
- total fuel quantity decreasing at an abnormal rate
- excessive fuel flow
by exterior observation we can have
-fuel spray from strut or engine