There are only two ways that I can think of to locate the exact source of a leak from the fuel tank:
(1) You can use a smoke generator to fill the tank with dense smoke, and then watch to see where it is coming out
(2) If you don't have acess to a amoke machine, about the only thing you can do is to remove the fuel tank for inspection to find the source of the leak. Some volvo models have access covers above the fuel pump; if your model has one perhaps this may give you a view of the top of the tank before removal.
In any case, if the tank is leaking you will need to remove it for replacement anyway. Repair of rusted fuel tanks is not a recommended practice; if it perforated in one spot, it is likely to perforate again nearby as the metal is probably rusted pretty thin. Soldering of leaks in a fuel tank can be a dangerous practice in itself.
I hope this is helpful; if so, an accept would be most appreciated! thanks!
Auto Service Technician
25+ yrs experience as a professional working technician; ASE L1 master technician