How can one denounce wrongdoing upon witnessing the precursors of adultery between a man and a woman in a public park?

Changing the reprehensible (munkar) can be an individual obligation (fardh 'ayn) or a collective one (fardh kifayah). If some people undertake it, it falls away from others, but if everyone abandons it, they all incur sin. If a person is the only one capable of removing it, it becomes an individual obligation upon him.

Changing the reprehensible should first be done with the hand; if that is not possible, then with the tongue; if that is not possible, then with the heart, and this is the weakest of faith.

If you cannot change it with your hand, then promptly use your tongue. If they do not desist, then separate them if you can, or threaten to report the matter to the authorities. If you are unable to do any of that, then denounce it with your heart.