Let's see if I can list a few heuristics for cheating:

• Mismatch between the writing style in different parts of the paper. If some paragraphs are poorly-punctuated and ungrammatical, and other parts are written in very formal academic language, that's a sign that the paper may have been made by copying and pasting from other people's writings, then filling in the cracks with their own writing.

• Formal academic language is a very weak warning sign. It means you should try typing some statistically unlikely phrases into Google, just in case.

• Sometimes people who are asked to summarize some assigned reading will do so by copying and pasting directly from it, and changing a bit of the wording around. This is pretty easy to detect if you've read it recently as well.

• If a completely incompetent student suddenly turns in top quality answers, it's unlikely that this is due to him just getting his act together.

I have no idea why your teacher thought you copied a paper for your economics class, but those are some heuristics that I've learned.