Use this when you think something is true, but you want to make sure:

Isn't this, like, your fifth marathon?

Isn't it considered rude to point at someone with your left hand in some countries?

You can also use this when someone says something and you think they might be wrong:

A: Edgar's coming to town this weekend.

B: Isn't it next weekend?

Note that, if the subject of the sentence is plural, you change it to "Aren't..."

Aren't you supposed to be at work right now?