How to check a 3D xform matrix is actually 2D?

[Michael Goffioul]

Hi,

This is not really my area of expertise, so I thought asking here
would be easier. I assume there are a few maths experts on this
list.

I'd like to find a way to determine whether the current axes
transformation matrix (4x4 projection matrix) is actually a 2D
transformation. In other words, I'd like to know whether an axes
object is actually in 2D mode (with any axes x, y, or z being
orthogonal to the screen).

I feel there must be a way to check that using some math
on the xform matrix, but I don't know what.

Note: the current axes transformation matrix is available in
the property "x_rendertransform".

Michael.