It's not easy.
A table has a property which I think is called "storyOffset" (need to
double-check the exact name) which gets you the insertionPoint in the
story where the table is. Then from there you will have to calculate the
width of all the cells, and borders, and add it all up to get the
horizontal offset. Quite a pain to do. Maybe somebody's got a function
out there which they're willing to share.
However, there may be a shortcut if you can create a temporary anchored
object, and insert it into the cell, and read the geometric bounds off