Which is the sign by we can tell that a certain karma has been definitely burned?
—Daniel , Argentina
You know this when that particular karmic action is no longer in your awareness. You simply don’t find yourself thinking about it anymore, and you don’t feel caught by the energy of it when you do think about it. In addition, you can simply not be aware of that karma as something that bothers you. In other words, the energy around that karmic action has been dissipated – usually by your own efforts as well as by the guru’s grace.
As an example, if you were bothered by a particular habit, you might find yourself waking up one day and thinking, “I haven’t acted in that way (or had that thought) in a long time!” It simply seems to be gone. There can also be a feeling of lightness and freedom around that thought or action if it does come into your awareness.
Karma means action, and a karmic action consists, in essence, of a vortex of energy revolving around a certain thought and action. Some actions and thoughts involve a lot of energy, while others are smaller. Dissolve that energy vortex, and you are dissolving the karma. Of course, the opposite is also true. If you engage again in that particular action that was finished, then you are creating a new karmic action and a new vortex of energy.
For further information I would recommend reading the Karma and Reincarnation book, from the Wisdom of Yogananda series.
I hope this will be helpful.