How are good and bad Karmas determined in Buddhism? One important thing that Hindus and Buddhists agree on is the law of Karma.  That is, there are certain actions which will lead a person to have a positive experience in future, either in this life or the next, and there are other actions which will lead a person to have a negative experience in future.  And Hindus and Buddhists also agree that ultimately the goal of life is not to just do good Karmas so you can have positive experiences in future, but rather to break out of he whole cycle of Karmas and their consequences.  Buddhists believe that this can be done through the Eightfold Path, and Hindus believe in other means like knowledge of the supreme being.

But my question is about what determines good Karmas and bad Karmas.  Hindus believe that the Vedas (and texts derived from them) tell us exactly what actions constitute good Karma and what actions constitute bad Karma.  But Buddhists reject the Vedas, so my question is, how do Buddhists determine what actions are good Karmas and what actions are bad Karmas?  Do Buddhist scriptures discuss how to determine this?

Note that I don't think the answer is "The Eightfold Path", because that's about what's required to break the cycle of Karma and consequence, whereas my question is about what's required to get positive experiences through the law of Karma.  Also, on a side note, do Buddhists simply classify Karmas into good or bad, or do they subdivide these categories further?  Because Hindus believe in a more fine-grained system where different kinds of good Karmas produce different kinds of positive experience.
Please see this answer on karma, rebirth and effects of karma on this life and future lives.

From the Nibbedhika Sutta:


  "Intention, I tell you, is kamma. Intending, one does kamma by way of
  body, speech, &amp; intellect.


In Buddhism, intention is the basis for karma. There are some guidelines on removing unskillful thoughts and qualities. The story of the blind elder monk Ven. Chakkhupala in Dhammapada 1 and the story of the hunter's wife in Dhammapada 124 further illustrates this.


  Dhammapada 1: All mental phenomena have mind as their forerunner; they
  have mind as their chief; they are mind-made. If one speaks or acts
  with an evil mind, 'dukkha' (suffering) follows him just as the wheel follows
  the hoofprint of the ox that draws the cart.
  
  Dhammapada 124: If there is no wound on the hand, one may handle
  poison; poison does not affect one who has no wound; there can be no
  evil for one who has no evil intention.


It appears that the simplest way to identify unskillful thoughts and actions, is whether they are imbued with the three poisons or not. And for skillful thoughts and actions, it's the opposite.

In the Vitakkasanthana Sutta, we find that unskillful thoughts imbued with desire/ greed, aversion/ hatred, or delusion have to be removed.

In the Vina Sutta:


  "Monks, in whatever monk or nun there arises desire, passion,
  aversion, delusion, or mental resistance with regard to forms
  cognizable via the eye, he/she should hold the mind in check.
  [Thinking,] 'It's dangerous &amp; dubious, that path, thorny &amp; overgrown,
  a miserable path, a devious path, impenetrable. It's a path followed
  by people of no integrity, not a path followed by people of integrity.
  It's not worthy of you,' he/she should hold the mind in check with
  regard to forms cognizable via the eye.


And the same applies to the other senses.

You can find a similar discussion in the Aparihani Sutta where it talks about removing "evil, unskillful qualities such as greed or distress".

In the Sacitta Sutta, also teaching us to remove unskillful thoughts:


  "If, on examination, a monk knows, 'I usually remain covetous, with
  thoughts of ill will, overcome by sloth &amp; drowsiness, restless,
  uncertain, angry, with soiled thoughts, with my body aroused, lazy, or
  unconcentrated,' then he should put forth extra desire, effort,
  diligence, endeavor, relentlessness, mindfulness, &amp; alertness for the
  abandoning of those very same evil, unskillful qualities.


Detailed classification can be found in the Abhidhamma in the volume called Dhammasangani ("Enumeration of Phenomena"). We can find a classification list in the work The Abhidhamma in Practice by N.K.G. Mendis:


  Sa"nkhaaraa is a collective term for the other fifty cetasikas (mental factors). These
  fall into four groups:
  
  
  Universal mental factors (sabba citta saadhaaranaa)
  Particular mental factors (paki.n.nakaa)
  Unwholesome mental factors (akusalaa)
  Beautiful mental factors (sobhanaa)
  


You can read the details on this page for the full classification of the 50 cetasikas.

There are also other classifications there like the different types of cittas (momentary states of mind and consciousness) - vipaaka, kusala, akusala, kiriya. They have association with roots (hetu, mula - see below) and feelings (vedana) etc.


  There are six roots. Three are kammically unwholesome (akusala); the
  other three may be either kammically wholesome (kusala) or
  indeterminate (abyaa-kata), depending on the type of consciousness
  they arise in. The unwholesome roots are greed (lobha), hatred (dosa),
  and delusion (moha). The three roots which are wholesome in some
  cittas and indeterminate in others are greedlessness (alobha),
  hatelessness (adosa), and undeludedness (amoha). Though these last
  three roots are expressed negatively they have positive
  manifestations. Greedlessness manifests as generosity and
  renunciation, hatelessness as loving-kindness, and undeludedness as
  wisdom or understanding.


The above are only the Theravada texts. There will undoubtedly be additional discussion on this, in the Mahayana texts.

You can find useful info also in Ven. Thanissaro's Kamma: A Study Guide.




  OP: I think skillfull vs. unskillful has to do with what will lead you to liberation vs. what will serve to keep you in Samsara.  But my
  question is about what will serve to give you positive experience in
  Samsara vs. what will serve to give you negative experiences in
  Samsara.


Skillful thoughts and actions will give you a positive experience in Samsara and could also lead you to liberation. Unskillful thoughts and actions will give you a negative experience in Samsara.

But liberation requires much more than skillful thoughts and actions. It also requires one to abandon all the fetters.

For e.g. by practising one or more of the Brahmaviharas (loving kindness, compassion, empathetic joy and equanimity) which is skillful, one could obtain the pleasant experience of birth in the heavans. But this is not permanent.