Ehh is correct. The only way to tell is to look closely at the vent. At that size, you should be able to see something. If time continues to go by without it laying any eggs, it's probably safe to assume it's male. IME, warmer aquarium water is synonymous with vent visibility. Either way, it's a nice looking fish.Male and female midevils are pretty much identical.
You have to wait until the tube becomes exposed a bit.
Just call it a whatever you want until it lays eggs or exposes it's wiener.