In the fight scene between Venom and Riot, Riot says
You have a strong host.
Just because Venom manages to push Riot off once, that should not mean that his host is powerful. He did not even see him earlier. How does Riot know this?
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Riot is a symbiote to Drake, and Drake knows that Venom is a symbiote to Brock. If you recall, Drake sent his henchmen to Brock's apartment in order to retrieve Venom. Since Symbiotes talk to each other, it's logical to infer that everything Drake knows, Riot knows. So, yes Riot knows who Venom's symbiote is.
Since the symbiotes must work as a team, a weak link would be an easy kill. So, in that respect, with Venom's easy evasion of Riot's attacks and his strong counter-attacks, Riot was making a comment meant to indicate that Brock/Venom had a strong bond and worked well together. I'm sure Brock's physical power (aka actor Tom Hardy's size) didn't hurt, but it was never really expressly mentioned in the film that he's a weightlifter or that he has any abnormal strength.
Venom, Riot, and the other alien entitities need to find human hosts in order to survive on Earth. It seems only natural that they would be able to determine one host as being preferable to another. They have the ability to heal (to a degree), but if they choose the wrong host, it kills the host and symbiote. This was illustrated by the lab scenes when the homeless "volunteers" were unsuccessfully paired up with symbiote aliens. So, it seems logical that these alien entitities would be able to (instinctively) tell which human offered the best chance for survival, thus giving them uniques insights into the humans' condition.