I just moved in to a house with roof heating cables. The outlet it is connected to in the garage has an indicator light which comes on in the down position. The indicator light is off when the switch is in the up position. Am I correct to assume I leave the switch up when I do not want the cables to work, and that the indicator light will show me in the down position that the heating cables are on?
There are switches with indicator lights that illuminate when on and other switches that illuminate when off. The former are called pilot lights, the latter, illuminated switches.
To determine which yours is, you need to look at the switch without the cover plate and determine which position is ON. While up is the convention for a single pole switch that is completing a circuit, it doesn't really mean anything, since a switch can be mounted upside down.
Most single pole switches have an up indication stamped on the metal mounting frame. If that is positioned up, the switch is connected when the paddle is up.
If there is no indication, you will need a circuit tester to determine which position completes the circuit.
I have a couple of these switches in my house. They do indeed light up when turned OFF. In one application in our house the switch works the room light and is far from one entrance, so it makes sense and helps you find the switch in the dark. The other works an outside light, so maybe it's just the last switch the electrician had that day and he installed it instead of a standard switch because I rarely need to find the switch in the dark. But for whatever reason it is done that way.
Now I don't want to mislead you and just flatly state that it has to be this way in your house, because it doesn't. So this is how I would reason through it (without equipment): as BIB mentioned the convention is that up=on. If every other switch in the house is wired that way (up=on)(besides exceptions like 2 switches controlling the same light) I would think it is safe to assume it is a switch of this type. -why someone decided you need to be able to locate it in the dark I don't know.
That said, I would second BIB's comment that if you can remove the faceplate and verify switch type that is definitely the best thing you can do.
On this avenue, (and this is the method I recommend) if you can not look at the switch and tell what type it is with the faceplate off (it may bear no markings on the front, etc.) you can buy a "Foreign Voltage Detector" (or FVD)for less than $20 at your local hardware store. You can use said detector, which is basically a wand type device with a light/beep to indicate that Electricity is present. Don't worry, you do not have to make contact with any bare metal, or remove the switch (probably), you can touch the tip to the plastic wire housing or just get really close to it with the tip of the wand.
Testing: Put the tip of the detector near the wire on one end of the switch and see if you have voltage indicated, and then check the other end/wire, if you have voltage indicated on both ends the switch is on. To verify: Flip the switch and test again. You should find that only one of the two wires indicates voltage. You have found the off position. Of course, if starting out you only have voltage indicated on one end then the switch is in the off position. To verify: flip the switch. You should now have voltage indicated at both sides. This is the on position.
Check out this link for FVD use if you like.
Be very careful working with live electricity! Take proper precautions. If you take it upon yourself to take the switch out of the wall because it is too tight to test in place - be sure to turn off the circuit breaker while you remove the switch (test with FVD to verify no voltage before really handling the switch out of the box), turn breaker on to test once the switch is out and in a 'safe' position - hanging away from anything it shouldn't touch (do I have to say "Be careful" while the circuit is live?). Turn breaker off again before putting it back together.... Don't get electrocuted.
Hope that helps! :)