sex is easy. If it has long fins, with a long tail with sharp edges, it's a male. If it has short fins, and has round tips on its tail, it's a female. Color is also more vibrant in males, but as there are albino paradisefish about, it can be hard to tell.
I'd recommend trying the pea thing, and monitoring how much it eats.
Actually. . . . are you missing any other fish? Maybe he had a very large snack while you were out.
And if the temperature is higher than about 75 degrees or so in the tank, that always puts my paradisefish "in the mood" so to speak, so you might want to check that. . . . if it's hot and there are bubbles in the tank, it's probably a male, if there aren't bubbles, you very well may have an egg-heavy female on your hands.