How will I know that this state of no suffering will last forever? Suppose I have reached a state of no suffering. Then what is the proof that suffering won't arise again ? How can I be sure that state of Nirvana doesn't change or decay ?
Theravada Buddhist Answer.


  How will I know that this state of no suffering will last forever?


A good and important question. The thing is that you can't know whether or not this state of suffering will last forever.

There are especially two (and probably more) important reasons for this.

The first one being that fact that the Buddha didn't want us to accept his teaching based on blind belief, hear-say or something similar. The Buddha's teaching is a practical teaching meant for us to realize through especially meditation practice.

The second reason dives into the nature of reality and how to understand it. There exists different types of realities; Conventional and Ultimate realities.

Conventional realities consist of concepts. What characterizes a concept is that it can be understood by hearsay, logical reasoning, inference or intellectual thinking. In other words, such concepts are not regarded as ultimate reality.

An ultimate reality such as Nibbana cannot be understood by the above mentioned methods. A person born blind can never know colors no matter how others may explain those colors. Likewise, one can never know toothache if one has never had them. 

In the same way, insight-knowledge and Jhana can never be known, unless one has attained them. Nibbana, the end of suffering, can only be understood by personal experience, ie. through correct meditation practice. 

Ultimate reality is only accessible for insight-meditators, those who attained Jhana and the noble ones.

Lastly, be aware that Doubt (Vicikicchā) is one of the Five Hindrances to Enlightenment. The antidote for doubt is; studying the teachings, finding a good teacher and vipassana meditation practice.

The second Bojjhanga, ie. the Investigation of Phenomena (dhamma vicaya) is also a direct remedy for Doubt, since it leads to personal experience of the Three Marks of Existence (tilakkhaṇa) and how the mind-body complex operates.