The hint that these creatures are parademons lies not just in the Knightmare Batman clip where they appear but in all of the other information given throughout the movie.
This dream sequence hints at the eventual existence of Darkseid, but does not confirm it until the parademons are seen as a supporting military force. However, the giant Omega symbol on the ground is a trademark symbol of Darkseid, so much so, his signature power is called the Omega-beam. Modern depictions of the character have included the symbol on his armor.
The appearance of parademons varies widely, even their creator (Jack Kirby) was not consistent with their appearance. Overall, they are monstrous humanoids capable of flight, either using technology or in some versions via mutation. The flying part was the most important aspect since they were Darkseid's first shock-troop to open any battle. Armed with ranged weapons, terrifying speed and a degree of superhuman strength, they were the first enforcement Darkseid would drop to pacify an enemy force. Their swarming patterns, the picking up and carrying away of troops as well as their fast attack style is reminiscent of their appearances in the DC Universe. There are very few other known characters with this signature appearance. With the Omega symbol and their sudden appearance, it was a strong indicator of their existence.
Scott Free attempting to escape parademons as envisioned by their creator, Jack Kirby.
When parademons were not acting as shock-troops they would also be employed as jailers or hounds tracking citizens of Apokolips who broke rules, committed crimes or tried to escape the planetwide surveillance of Apokolips. They would be provided as support troops for ground operatives backing up those troops with uncompromising air power.
Parademons will only obey strong leaders and rarely have moral problems as long as their leaders are visibly kicking butt. Should their leaders lose or they find themselves taking casualties, they have been known to abandon such leadership and escape via Boom Tube.
Steppenwolf, one of Darkseid's major generals in the New 52 reboot.
They would often be lead by a leader whose power would be the equal of any member of the Justice League. Beings such as Kalibak or Steppenwolf, whose fighting capacity equals or exceeds that of Superman or Wonder Woman, would make this invasion force almost unbeatable.
Batman v. Superman has made enough of an effort to hint at the existence of Darkseid, both through the dialogue ("The dinner bell has been rung, he is coming...") as well as through their visuals (released in the movie during the dream sequence as well as in later unreleased footage) showing a being likely to be Steppenwolf or some other clearly alien being in possession of what would appear to be Mother or Father Boxes.