this is kinda a stupid question but it is for the science fair. so, how do you tell if worms are happy
All living organisms respond to stimuli. I can assure you that most creatures react differently depending on mood. Therefore, to find mood change (or to tell when they're happy), you need to monitor their habits. If this is for a science fair, I would state your hypothesis as something similar to "I believe that if a worm feels happy, s/he will respond differently".