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One way would be to collect all your points in World Coordinates, use any three of the points to define a User Coordinate System [UCS -> New -> 3point], and then for each other point, translate it from its original World Coordinates into the current Coordinate System:
(trans testpoint 0 1)
and look at the Z coordinate of what that returns. If the test point is truly in the plane, the Z coordinate will be 0. If not, it will say how far off the plane it is, and you can compare that to your desired tolerance.
Kent Cooper, AIA 
@OceanaPolynom wrote:
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After making a UCS from 3 points I noticed that if I LIST the other points the Z coordinate seems to be the difference between the plane as defined by the 3 points and the elevation of the point that I LISTed. I didn'ty undeerstand the other part, (trans testpoint 0 1) is that a Lisp function? Is testpoint a variable with the point coordinates?
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Yes and yes. If the points are Point entities or Blocks or something, then yes, List returns their coordinates in the current UCS. Likewise, if they're vertices of a 3D Polyline or Line endpoints or 3DFace corners or something, you could use ID and snap to them, which will also report coordinates in the current UCS.
One advantage of doing it with an AutoLISP routine is that if they're entities that can be LISTed, a routine could just ask you to select them, and it could find all their locations/insertion points for you. Then you could, for example, instantly get a list of only the Z coordinates of each point relative to the plane, sorted in order if you like, or have it report just the greatest deviation from the plane, and identify which one is that most deviant one, or a lot of other things that could be determined. The information could be written out to a file, or Text could be put into the drawing, or a Circle drawn around the most-deviant object, or all such objects not within the desired tolerance from the plane could be put on a special Layer, etc., etc.
Kent Cooper, AIA
@OceanaPolynom wrote:
Thank you for your help. Is it possible to find trans.lsp?
John
More than possible: (trans) is not a routine using AutoLISP functions, but is itself a native AutoLISP function -- you have it already [assuming you're not using some form of Acad LT that can't use Lisp]. It (trans)lates points from one coordinate system to another. The 0 and 1 in my example mean to translate from World Coordinates [0] to the current UCS [1], but it has other options.
Kent Cooper, AIA
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