This generally requires two devices. One transmitting a signal and one receiving the signal. Until now we have a WRT54GL. So what is the other device?
Assuming you have a computer with Windows and built-in wireless card to make measurements you either check the signal strength shown in Windows (if you use Windows to configure the wireless) or use the software which came with the wireless card (if you use that software to configure the wireless) or you get a wireless scanner like NetStumbler.
No. You cannot do it without a wireless card on a computer, unless you install 3rd party firmware on the WRT54GL which does a site survey and shows signal strengths. You would also have to put both devices into ad-hoc mode for them to transmit something which the router could receive and measure. But I am not 100% sure if a normal site survey would show it.
Moreover, measuring in this direction probably won't give you an accurate view of the signal strength at the camera or printer. You would measure the signal strength from those devices received at the WRT54GL. The signal strength at the receiver depends very much on the strength of the transmitter and the gain of the antenna of the receiver. Now this of course is not symmetric: You may well receive a signal in one direction but nothing in the opposite direction.
The only real way to measure signal strength of a wireless access point is to use a wireless client which shows it to you. The access point itself won't be able to tell you that.
The signal strength also won't tell you how fast and reliably you can transfer data through the wireless link.
