It's intuitively something you assume without justification and hence just assert it without proof.
Typically you write all the premises of an argument at the start.
It's possible to phrase the argument in a way that avoid assumptions. Simply say that you are performing your reasoning in the context where those assumptions hold, meaning that you don't assume your assumptions hold but that if they hold then you can deduce this and that.
@Zophikel: Makes sense? Anyway I'm going off so I'll answer your subsequent questions next time I'm on Math SE.
If you can't find a valid argument, you can't tell.
Worse still, even if there is no valid argument from some premises to some conclusion, it doesn't mean that the conclusion is not always true given the premises, but this you'll just have to file away in your mind temporarily because you need to know much more than basic logic to understand this haha..