Is there a method that I can judge if any instance is located at a point?

If you want to check if a x,y coordinate is already "covered" by some instance , the procedure bellow is what you want

/**
Check if x:y coordinate is inside bBox
@return - boolean
 - t if x:y inside
 - nil if outside
 border cases (on edges) are considered inside 
*/
procedure( MyPkgIsXyInsideBBox(x y bBox)
 x >= xCoord(car(bBox)) && x<= xCoord(cadr(bBox)) &&
 y >= yCoord(car(bBox)) && y<= yCoord(cadr(bBox))
)

And here is how to call it

instId->bBox
((10.4125 0.85) 
 (11.7625 1.275)
)

x = 0
0
y = 0
0

MyPkgIsXyInsideBBox(x y instId->bBox)
nil

x = 11
11
y = 1
1

MyPkgIsXyInsideBBox(x y instId->bBox)
t

Thank you. But I'm afraid tha's not what I want.

Over 200x200 instances are supposed to be arranged in fhe form of array, but actually not. I need to check each lowerleft corner of the array if any instance is located on, which means >40000 instances will be examined by the procedure "MyPkgIsXyInsideBBox" for each corner point, and there're >40000 points. This is too much time-consuming.

It's dbInstQuery() that you'd want to use. This uses the OA region query to find objects in a particular area - in this case, instances. If you specify the start/stop levels as 0, it will find top-level instances, and you can specify the area as a zero-area box if you want to find instances under a specific point, specify the bounding box as list(pt pt).

Andrew