What proof is needed to convict someone of forgery? I.E. Counterfeit Cash?

I unknowingly received a counterfeit $20 bill and proceeded to try and make a purchase with that $20 at a local store.I was told it was counterfeit and I have since received a summons from the police for AGGRAVATED FORGERY!!! This is a CLASS B FELONY!! I would think for forgery they would have to find counterfeiting items such as more cash at my house, or the basic elements needed to commit this forgery. I had no clue that this was phony money since it came from somebody that was purchasing a reptile from me. I very quickly looked at the money to count and it stayed in my wallet until I went to the store.What do they need to convict me? What do you think will happen? I appreciate any good feedback cus Im seriously freiking out about this. P.S. I live in Maine if that makes a difference. It was just a single counterfeit $20 I received. THANK YOU!