Yes, iris colour is different - adult Greater Coucal have bright red eyes (as on this bird).
The other good indicator on this bird is the mantle (back) - this bird is very plain brown, whereas Lesser Coucal would normally show paler streaks on the centre of the feathers.
And habitat is always useful - Lesser Coucal is unusual away from areas of dry grassland (mostly on the hills in HK) and would be very unusual in an urban park. Greater is common in any shrubby area, especially lowlands, and would not be surprising in Kowloon Park.
