Because blood parrots are monomorphic, it is not possible to determine the fish's gender simply by looking at it. Instead, you must vent the fish. This is best done using wet hands to remove the fish from the tank.
Remove fish and examine its dorsal side
This provides a view of the genital region.
Locate the genital pore between the anus and anal fin
A larger genital pore indicates a female, and a smaller pore indicates a male. Additionally, males have a slightly longer, tapered genital pore, while a female's genital pore is shorter and blunter.
Return the fish to its tank, keeping it separated from other fish
Once you have determined gender, you can mark the fish or take notes so as not to have to ever repeat the process.