Your best bet is to:
1. Consult your owner's manual: This is the most reliable source. It will have a diagram of the fuse and relay box.
2. Look for a diagram on the fuse box cover: Many fuse boxes have a diagram printed on the inside of the cover identifying each fuse and relay.
3. Use a multimeter: If you can't find a diagram, use a multimeter to test the relays. Look for relays that are labelled "fuel pump" or have a similar description. You will need to know the voltage of your car's system (probably 12V) to test this.
Without a diagram, identifying it by sight alone is difficult. There are many relays in that box, and they all look similar. The best approach is to use a combination of the above steps.
