recognize in wich vector the cursor is using an inclinated vector

recognize in wich vector the cursor is using an inclinated vector

Im trying to make a condition to know in wich vertor my cursor is, so i
tried to ilustrate with an image but don't have the necesary reputation to
post an image, so follow this link and imagine its a tablet:
inclined vector
So i need a code to know in wich vector the cursor is (not coordinates)
when the event ACTION.DOWN. I know I must use if conditions and some
formula but can't get into it
I apreciate your help (please apologyze my english)