How can you prove that an objects has a positive charge on it?

Answer 1

You have to think positive!

1. measure a potential difference between a known source.
2. Charge a strip of paper with a negative charge using a known source and and the other strip from the test source and see if they attract or repel.

Also keep mind that positive an negative is a relative concept.

Answer 2

See if an object known to have a positive charge repels it, or conversely a known negative charge attracts it.