You are translating the tile's location using a some algorithm:
x = x0*(tileWidth / 2.0F) + y0(-tileWidth / 2.0F)
y = x0(tileHeight / 2.0F) + y0(tileHeight / 2.0F)
Where tileHeight and tileWidth are the height and width of the tile's selectable surface.
Where x0 and y0 are your tile x & y coordinates.
From here there are two ways to go:Reverse the Projection
You can construct a Projection Matrix from this (you can use Java's AffineTransform for example or implement this yourself (more optimal because all you need is a 2x2 matrix.)
Once you have a projection matrix for this system of equations, your can dot a vector with it, which will translate that vectors xy components to the respective screen-location. To do the reverse, you need to inverse the matrix and dot the screen xy components. If you zoom in/out and scale your sprites, you need to also scale your projection matrix.
Use a Color Map
Essentially, you translate the cursors X/Y location to a buffered Image that looks something like this:
You can then get an approximation of the cursors location by dividing it into cells. Then pick the colour map and test which colour it is over to determine exactly which tile it is over.
If you need me to go over more details just ask.