(From: Tony Duell (firstname.lastname@example.org).)
Firstly the health/safety warning. Toner, because it's a very fine powder, is (a) carcinogenic, and (b) explosive. However, provided you don't make clouds of the stuff, you should be OK. It can't be THAT harmful, as some photocopiers take 'loose' toner, and some printers have separate toner/drum/waste toner units.
I've fixed mechanical problems on the SX and CX cartridges. I also once moved a good drum from an empty cartridge to an almost-new cartridge with a scored drum (don't ask...).
There are 2 things you need to do to refill one - empty out the waste toner tank and (obviously) add some new toner.
The SX cartridge is easy to dismantle. Unclip a little triangular plate on the side, pull out 4 plastic pegs, and the case comes off. The plastic bung for filling the toner tank is obvious. To empty the waste toner, you have to take the cartridge apart, remove a plate (2 screws) and empty the toner into something. I worked inside a plastic bag last time I opened one of these cartridges.
These printers use a combined toner and developer. The toner mix must be magnetic for the printer to work at all.
I've seen bottles of loose toner for some photocopiers. I have not idea which brands would be suitable (if any), but I might experiment sometime.
As for inspection/testing:
Well, the main test is probably drum sensitivity (but this won't change unless you are swaping drums). Now, if you get that wrong, all that happens is that the image is too light or too dark. There are some little plastic clips on the side of the cartridge that operate microswitches in the printer. And there are only 3 settings!. Believe it or not, I've found some remanufactured cartridges that work better when these blocks have been moved - in other words, the drum sensitivity wasn't checked properly.
Everything else should be mechanical inspection - condition of bearings/gears, leaks, primary corona, etc. Nothing that's impossible to do at home. Of course it's rather different if you're selling remanufactured cartridges - in that case you need to be sure the parts will last another 4000 pages or whatever. But if you're fixing up your own printer, and don't care if you have to fiddle with something else in 1000 pages time, then there's no real problem.
About the worse that can happen to the printer is that the cartridge dumps toner all over the insides. I've had that happen - once. And it wasn't from a cartridge that I'd had in bits. It was from a remanufactured cartridge. That, together with the incident where a CX cartridge had had the door assembly incorrectly assembled, which caused damage to the transfer corona assembly, convinced me to avoid remanufactured cartridges that I'd not rebuilt myself, or at least checked.
You do need to avoid mixing waste toner (which will probably have a higher concentration of developer) with the new toner. Empty out the waste toner tank, but don't add the contents to the new toner. But if the toner you are adding is similar (or better still the same) as the original stuff, there is no problem from the residual toner in the cartridge.
Plenty of printers (Ricoh engine?) use 'loose' toner + drum/belt as separate parts. In those, you do add new toner on top of the old, and it all mixes up. Now, why should Canon printers be so different?