Emphasizing the sentences
How people in France emphasize the sentences?
For example in English : The weather is hot, isn't it? You understood, didn't you?...
Or positive like : This is not real, isn't it?
Yes. But we can still emphasise negative sentences. To translate your example, we could say: 'Non, mais c'est pas vrai ?' (the correct 'ce n'est pas vrai' is here replaced with the colloquial 'c'est pas vrai') or 'Ne me dis pas que c'est vrai ?' (an affirmative sentence used as a question).
Another unformal way to say "isn't it" can be "hein ?".
Il fait beau aujourd'hui, hein ?