They grow those same pointy feathers on their necks, but it is not so easy to see. lower down on their necks, see if you see any pointy hackle feathers coming in. Feel their pelvic bones to see if they are really narrow and pointy, close together. Big feet bones. More cartilege over the bridge of their beak. A thicker beak at the base, close to their face. Taller, braver stance. Put them to walk around and make a fake rooster sound, or a Scooby Doo sound like when he says Huh?, or a higher pitched whistle fading downward like a red tail hawk. See if they stand up tall and challenge the sound, or duck and hide.
No one thing will give you the answer, but more of them pointing toward roo, probably a roo.
1. Hackle/Saddle feathers- Yes= guaranteed roo
2. Pelvic bones, very subjective, usually not much help.
3. Bigger feet and leg bones- Yes= somewhat indicative, not "proof"
4. More cartilage over the brige of the nose, thicker beak.= often a fairly strong indicator
5. Tall stance, meh... small amt of help, might tip the vote but don't base much on it.
6. Challenging the sound of a hawk, if they challenge, probably a roo, if they duck, could be either way.
That's how I try to figure it out. Hope it helps.