How to know what is Dharma and what is Adharma?

How to tell if something is part of Hinduism or not?

Manusmriti 2.6 answers this question-

1. Vedas are source of whole Dharma

Vedas are above all others in terms of authority, If Vedas say something then its indisputable.

2. Smritis, the memories of people who have studied Vedas and their nature, by studying smritis one can tell what is Dharma and what is Adharma, but if contradictions exist among smritis or with vedas then whatever Vedas say is final.

3. Behaviour of good people tells us what is good and what is bad, that's why we have Itihasa, to study what happened and learn from it, we see what people did then and try to follow it.

4. What pleases self is Dharma-

this one needs a bit understanding, doing whatever we feel like is not dharma.

Deep inside we always know what is right and what is wrong, when we do something good, we feel good from inside when we do something bad we feel bad ....

But emotions, ignorance etc overpowers that and we end up doing bad things.

So if we hear the true inside voice, removing the ignorance and emotions, we ccan know what is Dharma and what is Adharma.

So to know is something is part of Hinduism-

1. Check with Vedas if Vedas do not support it then it's not Dharma

2. Check with Smritis(dharmashastras etc) if smritis support it then its Dharma
But if teachings if smriti contradicts with Vedas then Vedas are authority.

3. See Itihasa, wether good people did it, if they did then its Dharma

4. Listen to your heart do what it says, if you don't have access to any of above mentioned, but here we can be wrong due to ignorance.
(4th point applies fully to only those who have proper knowledge)