What clues does he recognize?

From the text:

"Well, the nigger unlocked the padlock when he went in, and he locked it again when he came out. He fetched uncle a key about the time we got up from table -- same key, I bet. Watermelon shows man, lock shows prisoner; and it ain't likely there's two prisoners on such a little plantation, and where the people's all so kind and good. Jim's the prisoner. All right -- I'm glad we found it out detective fashion; I wouldn't give shucks for any other way.