How can I tell whether two triangles intersect in 2D Euclidean space? (i.e. classic 2D geometry) given the (X,Y) coordinates of each vertex in each triangle.
One way is to check if two sides of triangle A intersect with any side of triangle B, and then check all six possibilities of a point of A inside B or a point of B inside A.
For a point inside a triangle see for example: Point in triangle test.
When we test collisions on polygons we also have a surrounding rectangle for our polygons. So we first test for rectangle collisions and if there is a hit we proceed with polygon collision detection.