Ben 10: Race Against Time

Taking place after the series, the story opens in Bellwood as a mysterious figure appears immediately starts destroying things. Ben Tennyson (Graham Phillips), in the form of Heatblast (voiced by David Franklin), confronts him. After a short battle, Ben seemingly obliterates the villain.The next day, Ben goes back to school, and has trouble adjusting to normal life again. After a bad day he gets bullied by Cash (Tyler Patrick Jones), JT (Tyler Foden) and two girls he tried to flirt with earlier in the movie resulting in Greymatter (voiced by Carlos Alazraqui) causing complete chaos at a diner where Ms. Dalton (Aloma Wright) works. Later he and Gwen Tennyson (Haley Ramm) go over some yearbook photos she took. In the background of one with Ben, Gwen enhances a blurred spot and reveals the same villain Ben defeated earlier. Max Tennyson (Lee Majors) identifies him as Eon (Christien Anholt), an alien the Plumbers captured almost two centuries ago. When he arrived, he was half dead and brought a device with him called the Hands of Armageddon, which would open a time rift to the alien's home dimension and unleash his race upon Earth if activated. They travel to the containment facility where Eon is supposed to be kept, only to find it empty and his guardian aged to near-death.Traveling back to Bellwood, Max takes Ben and Gwen to the location of the Hands of Armageddon, guarded by the few remaining Plumbers ranging from Ms. Dalton, Mr. Hawkins the postman (Jeff Jensen), Fire Chief Whittington (Michael Runyard), and Principal White (Robert Picardo). Eon has followed them and breaks into the facility, but cannot activate the device. When Ben attempts to use the Omnitrix, it malfunctions, glowing purple and refusing to activate. Eon attempts to kidnap Ben, claiming it to be a rescue, but Ben escapes. Eon manages to corner Ben, explaining that his race learned to control time itself, but trapped themselves by misusing their power. He claims that his fate is intertwined with Ben's. Eon is scared off by an old man (Sab Shimono) who happens to be another Plumber before he can elaborate. Grandpa Max decides it would be best for Ben to leave Bellwood so that Eon won't find him, but Ben bravely refuses and they both come to a deal where Ben will be monitored daily by a Plumber in disguise.The Plumbers, all around town, guard Ben around the clock. When Ben goes to the school gym to be alone, Principal White attempts to calm Ben's fears just when Eon arrives again and shoves White out of the way. This time, Ben is able to become Diamondhead (voiced by Daran Norris) and fight him off. Later on that day, Ben decides to lure Eon into a trap by purposefully leaving himself open, but this backfires and he is captured along with Gwen and Max.At the Plumber facility storing the Hands of Armageddon, Eon explains some of the background of the Omnitrix: Ben can only remain in his alien forms for ten minutes at a time, a failsafe to prevent them from overwhelming his personality. Eon knows how to deactivate the failsafe, and in doing so can resurrect himself through the Omnitrix. He does so, and Ben is transformed into a younger version of Eon. The older one retreats since there are now two of him. Reborn, Eon activates the Hands of Armageddon. Gwen and Max manage to free themselves and work to stop him. While Max tries to disable the time rift, potentially at the cost of his own life, Gwen appeals to Ben inside Eon. Ben successfully overcomes Eon, and with the help of the other Plumbers manages to save Max and disable the time rift, sending Eon's race back to their own dimension.Just when it seems like they've won, time stops for everyone but Ben. The older Eon reappears, angered at Ben's victory. Ben transforms into Wildmutt (voiced by Dee Bradley Baker) and fights him off, eventually knocking him into the Hands of Armageddon, destroying both the Hands and Eon. After doing an impromptu magic trick for the school talent show, using Wildmutt's timely de-transformation and a well placed stage curtain, Ben finally accepts being just Ben for a while. However, Max points out that aliens are persistent, and as the film ends, an alien ship, resembling Vilgax's, flies towards Earth.

Question: How did Ben know the villain had not been eliminated?
Answer: A yearbook photo