How to detect external mouse connected to T22 via dock?

[jkn]

Hi all
I have a T22 running Linux (Gentoo as it happens). I run it in two ways:

i) standalone, using the trackpoint mouse
ii) connected to an IBM Dock, in which case I use an external PS/2 mouse connected to the Dock. It's a Dock I, I think, BTW

In the latter configuration my external mouse runs fine, bus is over-sensitive. I would like to be abel to detect whether the external mouse is connected so that at boot time I can run the xset command to reduce the sensitivity. But I can't find a way to do this - the contents of /proc/bus/input/devices seems the same in both cases:
I: Bus=0011 Vendor=0001 Product=0001 Version=ab54
N: Name="AT Translated Set 2 keyboard"
P: Phys=isa0060/serio0/input0
H: Handlers=kbd
B: EV=120013
B: KEY=4 2000000 3802078 f840d001 f2ffffdf ffefffff ffffffff fffffffe
B: MSC=10
B: LED=7

I: Bus=0011 Vendor=0002 Product=0006 Version=0000
N: Name="ImExPS/2 Generic Explorer Mouse"
P: Phys=isa0060/serio1/input0
H: Handlers=mouse0
B: EV=7
B: KEY=1f0000 0 0 0 0 0 0 0 0
B: REL=103

and I don't know of any other way to detect my external mouse.

Pointers please?

Thanks
Jon N