I know nothing about electrical engineering however... Is there a way I can detect when an electrical appliance is on? For example when I turn on my tv, a small blue light turns on so I know, but say I have a device that doesn't show it is on, such as a printer, but there's no display. Is there something (a component) I could put that can sense when something is using power? And where would the best place be to place such a device?
Yes. A current transformer properly applied, preferably to measure the differential mode current thru your device, would be the primary component. That produces a low voltage and isolated signal proportional to the current drawn by the device. You then threshold detect that signal, and use the result to drive something that turns a LED on or off.
The details of how to do this are beyond a reasonable answer here for someone that "knows nothing about electrical engineering". This is like trying to explain in a few written paragraphs how to drive a car, when the explainee has never even seen one.
At your level, you should just go out and get some off the shelf device that does what you want.
There are smart outlet strips that turn on the remaining outlets when the appliance plugged into the first outlet is drawing above some minimum current. They are available for USA 120 volt, 60 Hz power. If you want something like that for another power spec, you can search.
If you want to modify the appliance, you can find a point inside that is energized when the appliance is on and attach a light. If you are qualified to do that sort of thing, you can figure it out.
Maybe something like a power meter would do the trick. When your appliances are on or being used the current draw would increase. So as long as you test it for the specific appliance (ON/OFF) then you should know what the ballpark number is for when they're ON. (There may be some negligible current draw still for specific appliance even when they're "OFF") I'm thinking something like this could do the trick: http://www.amazon.com/TS-836A-Energy-Voltage-Electricity-Monitor/dp/B00E945SJG Hope this helps