Looks like a Koi to me. As others have mentioned, the only true way to sex one is to wait until they've paired off and spawned. The male's breeding tube is pointy while the female's is larger and more blunt. Obviously the female will be the one laying the eggs too.
I have heard that mature males can develop a hump on their head. Not ALL Angels will have this hump, so it's not a reliable way to sex (i.e. if it doesn't have one you can't just assume it's a female), but if there is one then it's a male.
Well as stated above im not that bothered about its sex as "it" will be on his own and will not been with another male or female as dont have room for the spawn and dont want them getting too aggressive. Is Koi the morph ?