Jaxon

While Jaxon wasn't able to say for sure what Alex's ability was, he was able to make an educated guess based on what he had seen so far. He had seen Alex swat his ice shards out of the air without touching them, twice, this lead him to believe that Alex was using an ability that allowed him to move things from a distance.

It might seem obvious that telekinesis was the answer but there are other ways to achieve a similar effect, for example, if someone was able to control wind then they could have swatted the ice shards away with a controlled burst of air.

Although there were other possibilities, the most obvious ability was still telekinesis and so Jaxon operated under that assumption going forward. When Alex mentioned a knife stabbing him from behind, Jaxon knew what was about to happen and immediately reacted.