You have at least two options:
you have the x:y coordinates of the point, just create a small bbox , like (x-0.001:y-0.001) : (x+0.001:y+0.001) and then use
[ tx_layer | t | nil [ lx_level ] ]
[ g_doRowCol ]
=> ld_fig | nil
If your object is overlapping this small bbox then it shuld be found in the ld_fig list.
Take care: ld_fig is "a list of lists [of lists [of lists [....]]] of db_objects" (nested lists).
If you know the x:y coordinates of the point and also the coordinates of your shape as poligon points you can use an algorithm like "ray casting" , see http://en.wikipedia.org/wiki/Ray_casting_algorithm#Ray_casting_algorithm .
You have to deal with the points are are on the perimeter: are they in or are they out ?