It sounds like you want to do a Job Safety Analysis. ;)
More seriously, there are really two questions here:
- Are there any hazards to my personal health and safety when dis-assembling consumer electronics?
- Is it safe to re-use the parts thus obtained?
Disassembly
Let's assume that you are sane, and you are never going to work on live devices. That means, at a minimum, that you have isolated the mains power, i.e. the mains power cord is entirely removed from the power outlet.
The major hazard here is so to with stored energy. Some parts of electrical devices can store enough charge to give you an electric shock. The large electrolytic capacitors in power supplies are good examples of this. Flyback transformers, as found in cathode-ray TV's, are also notorious for this.
The remedy for this is to discharge the stored energy.
Other hazards are more mundane. For example, eye protection was mandatory in our electronics labs, because side-snippers (wire cutters) can propel bits of wire at high speeds. Closed shoes were also mandatory in case you dropped a soldering iron on your foot. (I wear steel-capped safety shoes whenever I'm out of the house, so this was no great imposition on me.)
If you're working with a soldering iron, work in a well-ventilated room - the lead fumes aren't such an issue, unless you're doing it for long periods, but some of the things you may be burning could produce toxic byproducts.
Wash your hands after you're done. I was inducted to work in a lead smelter, and it gave me a healthy paranoia about heavy metal poisoning. Washing your hands costs nothing, and better safe than sorry.
Using the parts
The electronic device was designed to be fully safe when used by normal people. When you disassemble it, you are circumventing the design features that made it safe.
In the particular case of laser devices, like optical disc readers, the device was made safe either by limiting the output power to a safe level (so it doesn't matter if you are exposed to it), or by means of containing the laser energy within the enclosure of the device (so you can never be exposed to it.) If you take out the laser and use it for something else, you would have to apply similar care. Note that infra-red or ultra-violet lasers may be invisible!
In general re-using parts from disassembled devices is OK, but be careful what you use them for. I.e. when building anything to do with mains voltage, the parts involved must be rated to withstand voltages significantly higher than normal mains voltage, in case of surges on the power line. This is why power supply capacitors exposed to mains voltage must be safety rated X1, X2, Y1, Y2.
If you ever have a question about re-using a specific part, ask a question here and we can provide specific advice.
Best Answer
To test the triac out-of-circuit you can use an ohmmeter on the lowest range. You need to find the datasheet. If it's the type with a built-in diac you won't be able to easily test the gate with low voltage.
Test for continuity between MT1 and MT2, in both directions.
If you find continuity in either (or both) direction(s) then the triac is knackered, end of story. This is the most likely failure mode. It is typically caused by an overcurrent surge even such as arcing during the failure of an incandescent bulb.
Another test that can be done is to hold the meter terminals (using the lowest ohms range) on MT1 and MT2 and momentarily touch MT2 to gate without breaking either of the connections. The meter should show continuity as long as the gate is touching, and may continue to show continuity after the the gate connection is broken. You can test it both ways.
A more functional test that is a bit more trouble to set up is to get a 12VAC transformer and a 12V few-watt light bulb (maybe an automotive bulb) and connect the triac in series with the bulb to the transformer secondary. Use a 100 ohm resistor to momentarily connect gate to MT2. The light should go on as long as the gate is connected. If the light does not turn on or the resistor gets really hot the triac is bad. If the light goes on without the resistor (full or partial brightness) then the triac is dead.
Personally, I would order the triac immediately without testing. There is no other plausible failure that will cause the light to remain on at full brightness, and the triac failure is likely the only failure.