What makes us think charge is
different - that there are both positive AND negative charges?

Because the  force between them can be either attractive or repulsive

Explanation:

By convention, we assign two different signs to electric charges:

- Positive (e.g. protons)

- Negative (e.g. electrons)

Of course, this is solely a convention: in fact, we could have called "negative" the charge of the proton and "positive" the charge of the electrons.

However, we do know that there exist two different signs of charge. We know this because when charge are brought close together, they act in 2 different ways:

- If the charges have same sign (e.g. proton-proton or electron-electron), they repel each other

- If the charges have opposite sign (e.g. proton-electron), they attract each other

Therefore, these 2 different behaviours tell us that there exist 2 different signs of charges.