Make xmessage windows pop up on the current screen [on hold]

I use xmessage in a script called from a udev rule. I've seen here and there the recommendation to put the following at the beginning of a script using xmessage:

export DISPLAY=:0
export XAUTHORITY=/home/gauthier/.Xauthority

The problem is that if I am the second logged in user, DISPLAY must be :1 for the window to show.

How do I make the xmessage appear on screen, whatever the number of logged in user, and whichever user has its screen showing?