I'd use one of the many LineIntersect functions in the code archives with one line being your test line and the other being a line from the points old position to it's new position. If the lines intersect then the line was crossed. Also these functions return the point at which the lines cross.
Yet Another Line Intersect Function
Concave/convex polygon collisions and other useful functions.
Lines_Intersect()
There's probably a couple more too if you have a search.
Yet Another Line Intersect Function
Concave/convex polygon collisions and other useful functions.
Lines_Intersect()
There's probably a couple more too if you have a search.