Thx dieselnutjob I was unaware of that.
In which case citroDane if your looking at a 16V HDi (highly probable given the age) then it will have the 4HP20, good news
As for visual indications, the 4HP20 has a identification plate on the top towards the front right (when looking under the bonnet), you will probably have to remove the air box to see this plate, it should clearly have the ZF logo and say 4HP-20, plus a load of identifying numbers.
The AL4 probably has something similar, will check my car later.
Also under the bonnet the heat exchange unit looks quite different, the 4HP20 being most distinctive for its two double circles in a raised oblong on top of the gearbox, you can clearly see on this image.

Also shown in that picture is the identifying plate. The shiny metal cover is the front of the gearbox, that will face the front of the car.
The AL4 heat exchange is a rough horseshoe shape, situated at the back of the gearbox, not easily visible form top, but you can see the pipes going in and out.
If you cant find this under bonnet, its possibly obscured with airbox or other items, you could check underneath. Both gearboxes have different drain plugs, the 4HP20 has a drain plug, where as the AL4 as a plug within a plug style, see this photobucket album, this is a AL4 on a C5.
http://s168.photobucket.com/albums/u171 ... roen%20C5/
Not my photo's thanks to jackafrica who ever you are.
I hope this helps.