How do we know we are doing the right thing?

This is a personal dilemma.
The Buddha said, “Do good; do no harm; train the mind.”
This tests our conscience.

It all depends on whether we talking conventionally, absolutely or both.

On a conventional level, the right thing is not to create harm and to do good, bringing about balance: love.

On an absolute level, pure awareness recognises what it is that creates harm: in that, balance is automatically restored: love.

On a unity level, the right thing is expressing the ultimate in the relative: unconditional love.