The intention (volition or "cetana" in Pali) becomes your kamma as Buddha said in the Nibbedhika Sutta: Penetrative
"Intention, I tell you, is kamma. Intending, one does kamma by way of body, speech, & intellect."
You can apply this to your own life – your past, present & future. For instance you will see inequality in comparing yourself with with your friends and loved ones. In seeing this through Dhamma, Cula-kammavibhanga Sutta: The Shorter Exposition of Kamma will come to mind. Buddha said in this sutta:
"Human beings are owners of kammas, heirs of kammas, they have kammas as their progenitor, kammas as their kin, kammas as their homing-place. It is kammas that differentiate beings according to inferiority and superiority."
This sutta suggest that habitual tendencies of the mind (gathi) play an important role in what you are in your past, present & future. when one thinks, speaks, and bodily acts, a trace of those thoughts, speech, and actions are recorded (like a tape) in the mind plane. On the other hand, the kammic energies associated with a person’s activities are also recorded in the mind plane as kamma seeds having kammic energies associated with them. The real danger in building up bad habits (gathi) is that one could progressively get into worse habits. A teenager who starts drinking could then start using drugs; then it could lead to hanging out with even worse friends and get into drug dealing or even killings. To break away from bad gathi, one needs to make a determination not only to stop such bad activities but also to build up the opposite good gathi, and start heading in the right direction.
If ever you live long, it is due to wholesome kammas before his previous birth unlike that of a person who has engaged in a lot of killing in a past life. However it is important to note that these are guidelines (not rules) and no one can predict the exact destiny of birth of someone. Kamma Vipaka (results) therefore is a very complex subject with many mechanisms in action. Now we can see why Buddha categorized this as a one of the "Acinitita" subjects in Pali.
When you ask How do you know if you have bad/good karma, what you really want to know is, as to "why do bad things happen to good people?" Being a part of this community, you are sure to do wholesome kamma, and rightfully you expect good results in return. If you are a "good person", then you should not get "bad results". However it will not be so. Bad things are sure to come your way. How could this be explained? Again in referring to the scriptures, you will find three types of kamma with regard to the time of their results:
kamma resulting in this life-time (ditthadhamma-vedaniya-kamma),
kamma resulting in the next life (upapajja-vedaniya-kamma),
kamma resulting in later lives (aparapariya-vedaniya-kamma).
The reason for bad things to happen to good people is not only due to Kamma. It is not the sole determining factor. Buddha's teachings explain that there are five orders or processes (Niyama Dhamma) They are enumerated and defined in the Dhammasaṅgaṇī-aṭṭhakathā:
- Utu Niyama(Physical-Seasonal changes and climate)
- Bija Niyama(Biological/Genetic inheritance)
- Kamma Niyama (Ethical /Consequences of one's actions)
- Citta Niyama (Psychological/Will of mind)
- Dhamma Niyama (Laws of nature).
What we can learn from this is that what we experience in life, day in and day out, is the result of a complex interactions of physical, biological, psychological, ethical, or laws of nature. Kamma may play a predominant role, but what ripens is a result of the other four conditions coming together. Therefore kamma is only one of the five factors that come into play when something happens to us, either good or bad.