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,
The Pali suttas do not literally state this. The Pali suttas (eg. AN 6.63) state there are three results of kamma: (i) now; (ii) later; (iii) later again.
For example, the kamma of drinking alcohol has three results: (i) pleasure now; (ii) headache tomorrow; (iii) habit, craving or addiction later.
Next life kamma is an interpretation rather than a literal reading of the Pali suttas. In the Pali suttas, words such as 'birth' ('jati'), 'death' ('marana'), 'body'('kaya'; group) do not necessarily have a physical meaning.
The Buddhist commentaries & suttas (somewhere) state how the ordinary person views Dhamma is the opposite to how an enlightened person views the same Dhamma teachings. In the commentaries, this is known as the 'two truths'. The ordinary person reads everything as physical materialism where as the enlightened person reads everything as it really is. Thus, the Buddha does not speak lies when he uses language that can be interpreted in two-ways.
In is not correct to assert Buddhism teaches about past & futures lives because this is not literally said in the original scriptures (although is literally asserted in later scriptures & commentaries).
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.
The Buddhist idea seemed to exist systematically before the Hindu idea. At least the idea of moksha with Brahma appears to be not mentioned in the Pali suttas, which report discussions with Brahmans who followed the Vedas. While the early Upanishads are not mentioned in the Pali suttas, it appears the ideas in them about moksha seem to be either immature or not widespread at the time of the Buddha.
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.
The Buddha did not reject anything in the Vedas he agreed was beneficial. The Pali suttas explicitly state the Buddha was a friend of the Brahmans and wished no harm to the Brahmans. When the Brahmans tried to argue (like Jews) they were a chosen or superior caste, born from Brahma's mouth, the Buddha replied if they do bad kamma how can they be superior?; to which the Brahmans agreed. So both Buddha & the Brahmans had respect for good kamma as a determinative factor for a superior 'birth' (status).
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?
In Buddhism, the ten skilful kamma are non-killing, non-stealing, non-sexual-naughtiness, honest speech, pleasant speech, cordial-non-divisive speech, beneficial speech, non-greed, non-hatred & non-delusion. Apart from this, there are many other lists, such as in the Sigalovada Sutta.
In Buddhism, everything is determined on the basis of causing suffering/harm & leading to freedom from suffering/harm, which can be known via personal experience. Refer to the Kalama Sutta.
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
Also, on a side note, do Buddhists simply classify Karmas into good or bad, or do they subdivide these categories further?
Basically two-categories, namely, wholesome/skilful & unwholesome/unskilful (although there is a sutta with four types: white, black, white-black, neither-white-black).
Naturally, there are more refined superior good kammas, such as excellent generosity, meditative heavenly jhana, loving-kindness (metta), serving arahants, etc.
For example, there are many different godly realms in Buddhism, which are the results of different kammas as well as human, animal, ghost & hell birth from different kammas.
Godly realms represent special attainments, such as worldly wealth, political power or higher spiritual qualities, such as psychic powers.
Human birth means being moral.
Animal birth is being immoral, ghost birth is being chronically craving & hell birth is suffering & torture.
Because Hindus believe in a more fine-grained system where different kinds of good Karmas produce different kinds of positive experience.
Buddhism is an experiential tradition. The degrees of kamma are known in the heart.