isometric tile recognition

Hi,

im attempting to create an isomertic game in vb using the glorious BitBlt. my problem is a math one, i hope you can help.

i have a series or isometric tiles in the game area, the problem is at the moment i wish to be able to identify which tile is being clicked on the bitmap. i am certain that there is a caculation i can do to work this out but im having a total brainfart. please help or point me in the right direction. thanks :D