First of all I apologize for my bad english, I hope that you will understand me.

I'm working with documents which have very wide tables and I need to put them on a landscape page but I don't know how to detect if they are overflowing the page width or not.

I know how to use named pages to put all tables on landscape pages but I don't find a way to put only the tables which have a bigger width than the page width.

I can't manually modify the source document to add a class to the wide tables, I have to detect their width automatically...

Could you help me ?

If there is anything you don't understand about my question, please feel free to ask.

Currently this is not possible, unless you can detect the width by the number of columns, or something like that. Because the actual physical width is not known until after typesetting, and then it is too late.