I think he means a tower defense game and the "defense unit"-creation issue.
Oops, forgot to answer the initial question! (amazing smiley, so OMC-ish)
A simple way of doing it is using an active object (identical to the unit you are creating) following the mouseXY as long as the mouse button is pressed(When it's not pressed, simply hide the active somewhere).
Use that active to check if the current position is free(i.e if the active is overlapping something or not).
If the position is free + player releases the mouse button, simply create the real unit at the active's current XY, if the position isn't free - hide the active.
For different units, simply use multiple animations or directions for the active following the mouseXY.
It's a lot more simple than it may sound, but I'm sure you get a hang of what I'm talking about.