Watch the episode until about seven minutes in. At about this time, Krillan, Tein, Yamcha and Trunks are all knocked to the ground. Only Trunks stands up.

As he tries to stand up, the slip-up is obvious.

Trunks's pupils in his eyes are visible.

That's not possible considering he is in Super Sayan form,plus we saw his eyes at close up when he fought Freiza.