Trixie and Spud find out.

After Trixie and Spud sold Jake to Rotwood for cash, They go to Jake's house to tell him the news. After Haley let them in his room, they pondered on where he could be. When Spud sat down, he turned on the computer and they both spot images of Jake as a dragon and Jake Forgot deleted the images from his computer.They even had images of him "dragoning up". When they found out they realized that they sold Jake to Rotwood for cash.

Example of:
Wham Episode