help.. intersection..

i have an edge (or a line) given by two co-ordinates (x1,y1,z1) and (x2,y2,z2). now i want to know how to check if this line/edge is inside a frustum, i mean both the points could lie outside the frustum and it can still be seen. i dont want to clip the line, all i want to know is if it would be seen or not, i.e it either passes thru the frustum or lies outside. thanks