Check out this simple and detailed rubric which is to be used to judge, evaluate, and score Science Fair projects. For transparency and clear expectations, I would suggest sharing the rubric with students before their projects, so they know what they need to be successful.
Also, check out my Science Fair Poster Board Expectations.
You can also find my Science Fair intent to participate submission form as well.
Lastly, you can find my Science Fair Peer Scavenger Hunt Activity!