How can you determine if a polygon is a convex?

No side of the polygon crosses any other, even when extended indefinitely.

Step-by-step explanation:

If no sides of the polygon cross each other, it is convex when it has no internal reflex angles (angles measuring more than 180°). If sides of the polygon cross each other, it is not convex.

Another way to describe a convex polygon is that extending any one side indefinitely will not cause it to cross any other side.