Due to the nature of the game it is nearly impossible to tell if an item was cheated in. The only way to know is if the world they're in is incredibly fishy like chests upon chests loaded with 3000 plat coins.
Hey foxfur, You ever heard of the screenshot tool in terraria?
You press f1 and it allows you to take a screenshot. It even takes you to the folder it is in if you are in windowed mode.
Then you can upload the file to imgur or here and have a higher quality screenshot