How to distinguish Window and Pixmap ID

[Ren, Zhaohan]

Hi all
Windows and pixmaps together are known as drawables. Now I get a "drawable" ID(Created by XCreateWindow or XCreatePixmap) , I want to know how to distinguish them, that is to say how to distinguish if a "drawable ID" is a pixmap ID or window ID.
It seems that I didn't find any Xlib interface to achieve this. Are there any good method to use?
Thank you

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