I‘ve ATI Radeon VGA [X1550 64 bit] card. How do I find find out if the driver is installed or not under any Linux distribution running xorg server?

You need to use following commands and files to verify if driver is installed or not:
[a] lspci – list all PCI devices including your display adapter

[b] lsmod – program to show the status of modules in the Linux Kernel

[c] /etc/X11/xorg.conf – xorg X Window System server configuration file

lspci example

First, find out if card is detected by Linux or not, enter:
# lspci
# lspci | grep -i radeon
Sample output:

01:00.0 VGA compatible controller: ATI Technologies Inc RV505 [Radeon X1550 64-bit]

Next make sure driver radeonhd is installed and configured:
# dpkg --list | grep -i radeonhd
If driver is not installed simply download and install it from x.org web site (look for xf86-video-radeonhd driver) . Finally, make sure driver is installed:
# grep radeonhd /etc/X11/xorg.conf
Sample output:

        Driver          "radeonhd"

radeontool command

You can also use radeontool command to see if radeon is installed or not:
# radeontool --debug
Sample output:

01:00.0 VGA compatible controller: ATI Technologies Inc RV505 [Radeon X1550 64-bit] (prog-if 00 [VGA controller])
        Subsystem: PC Partner Limited Unknown device 0840
        Flags: bus master, fast devsel, latency 0, IRQ 11
        Memory at 80000000 (64-bit, prefetchable) [size=256M]
        Memory at 90300000 (64-bit, non-prefetchable) [size=64K]
Radeon found. Base control address is 90300000.