Visual inspection of a Powerbook can reveal a lot about the Powerbook and what it supports:
Powerbook G4 with side USB ports as opposed to rear under flap:
Supports Airport Extreme (802.11g), WPA2 encryption. Supports up to Mac OS X 10.5.8. Does not support booting into Mac OS 9, but does support Classic if you have the original installer discs or a clone of the originally installed software.
Additionally, this image can tell you if it has Firewire 800 support by showing you what the ports look like:
September 2003 and later Macs if so identified by the name of the machine inside the battery bay also support USB 2.
Powerbook G4 with rear USB ports:
Suppors only the original Airport (802.11b), with at best WPA1, and WEP encryption. If you are within 1500 feet (500 meters) of someone who can intercept it, wireless is best done with an ethernet 802.11g wireless bridge, such as Apple's current Airport Extreme and Express base stations. Inside the battery bay it will tell you if a DVD drive is present. If there is, it can support at least up to 10.4.11. An external Firewire DVD also will support booting from 10.4 if it is make by certain manufacturers. The original CD exchange program retail version of 10.4 is another method of bypassing this issue, if you can find those CDs. If it is 867 Mhz or higher it can support up to 10.5.8 (if there is a DVD drive as well), and dual booting with Mac OS 9 if you have the original discs, even though 10.5 and later do not support Classic. If it is 15" (38.1 cm) diagonal or larger screen (not including the frame), it also supports PCMCIA cards. Target Disk mode is supported to treat the Powerbook like an external Firewire hard drive for other computers. Note: this does not mean it can always bypass the system limitation of other computers.
Powerbook G3 with Firewire - supports booting from external Firewire drives, and Target Disk mode. Supports up to Mac OS X 10.4.11 if there is a DVD drive present. If there is no DVD drive present, only supports up to 10.3.9, unless you are lucky enough to have an external Firewire by certain manufacturers or the original CD exchange program CDs mentioned earlier. The drive/battery bay if filled can be ejected to determine what type of drive is inserted. Target disk mode is supported (see previous computer type for more info).
Powerbook G3 with USB and SCSI (a 5 x 6 row of thirty pins). Supports up to Mac OS X 10.3.9 if the CPU supports it. Some reports of these machines have had difficulty installing 10.3 or later. SCSI booting is supported if the jumper of the external drive is set correctly. SCSI disk mode can treat the computer as an external SCSI hard drive to other computers.
This is only the first draft of this tip. Any additions, feel free to add.