Guide for diagnosing problems?

Is there a definitive guide for diagnosing problems?

I have installed XFCE in the past and run it successfully, but most of the time I just use a CLI environment. Now I'm trying to run XFCE on a ThinkPad X61. In /var/log/Xorg.0.log
I see:-
(EE) open /dev/dri/card0: No such file or directory

/var/run/dmesg.boot contains:-
agp0: <Intel GM965 SVGA controller> on vgapci0

Do I need to install a specifig video driver to get this working?
(EE) open /dev/dri/card0: No such file or directory
I do believe you almost always get that message. Even when things are working correctly.

Look through the log file, look for the hardware it detects and what exactly it detects. It generally helps if you look through a couple of logs from systems that are known to be correctly working. With some experience you can usually spot things that are out of place. That's usually a good indication if it's working or not.
I would install the vesa driver and see if that works.
pkg install xf86-video-vesa
(If it does work, it may be unbearably slow though.) If so, it's an issue with the Intel card, some of which may work with CURRENT and the drm-next stuff.