How do you tell which one is action force and which one is reaction force?

Tough. An action force is normally the aggressor, acting first, with a clear owner (for example, a human, an animal, an engine, etc). The reaction force is normally passive (only acts when being forced to) and hidden (invisible), with no clear ownership. The reaction force can be created artificially to explain the situation.

An example of a reaction force: an apple is sitting still on a table. We know the apple will fall if the table is not there. The action force will be the gravitational force. We can explain why the apple does not fall to the floor by 'assigning' an invisible force by the table that is equal and opposite to the gravitational force. Otherwise, Newton's 2nd and 3rd laws of motion would have been violated.