To find it definitively, you'll need either:
* A wiring diagram: This is the best approach. The diagram will show exactly which relay is the fuel pump relay and its location within the box. You can find these diagrams online through Volvo parts websites or forums dedicated to classic Volvos.
* A multimeter: You can use a multimeter to test the relays to see which one controls power to the fuel pump. This requires some electrical knowledge and caution when working with car electrical systems.
Without a diagram or multimeter, simply looking in the relay box is likely to be insufficient for positive identification.