If you are experiencing mental Dukkha, then you have normally just had an unwise thought or said or done something unwise. If you are experiencing mental peace, harmony and contentment, then you have normally just had a wise thought or said or done something wise. Or possibly you are just deluded and you don't know what you thought. But whenever you are actually having mental Dukkha, it means you have done something wrong. You have not had a right thought, you have not had right speech, you have not done a right action and that is where your mental Dukkha is coming from. So you can look straight at your thoughts and see, "Ok, what are my thoughts right now? Am I experiencing mental Dukkha or am I not?" If you are experiencing Dukkha then you can think, "Ok, why? Why am I experiencing Dukkha?" This is the Four Noble Truths; you have Dukkha, where did it come from? It came from you doing something wrong in your actions, speech or thoughts. So as to recognizing right and wrong thinking, speech or action, see whether you are creating Dukkha, especially in your own mind.
Now, are you harming others? This is another way to judge. Harming others is connected with your speech or actions, it has nothing to do with your thoughts by themselves. You must actually say something or perform an action in order to harm others. Are you harming others? Do you see that what you did harmed others? In general, whenever you are harming others, you have done something wrong.
Now, harming others must be looked at in a broad way. If I have a splinter in my arm, and I go to you and ask you for help, "Please get the splinter out." And you take a needle and you poke in my arm and you pop it out. You have harmed me initially, it hurts to get that needle in there, but in the long run my arm can now heal itself. So that type of harming others when you are just harming momentarily in order for more help in the future, that is not what I am referring to here o.