Is my Motor/Speed Controller/Battery Pack Too Hot?
 

Although not always true, a rule of thumb is:
Hot motor, cool controller, cool batteries = undergeared
Cool motor, hot controller, hot batteries = overgeared
Warm motor, warm controller, warm batteries = happy components

Hot, warm, and cool vary as well, so here's another general rule:
Hot for a motor = generally over 160F (71C)
Hot for a controller = generally over 150F (66C)(or "thermal" range for the controller)
Hot for batteries = generally over 150F (66C) for nimh, 130F (54C) for lipo
Warm for a motor = generally 100F (38C) through 150F (66C)
Warm for a controller = generally 100F (38C) through 130F (54C)
Warm for batteries = generally 100F (38C) through 130F (54C) for nimh, ambient temp through 120F (49C) for lipo
Cool for all components = generally ambient temp and up to 10 degrees F (5.6C) more

The reasons the above are not always true are numerous, but may include:
Sticky drivetrain - will put un-necessary strain on electronics whether geared correctly or not.
Poor quality batteries - batteries will heat up regardless of gearing.
Wrong motor for the application - will put un-necessary strain on electronics whether geared correctly or not.
Wrong controller for the application - will put un-necessary strain on electronics whether geared correctly or not.

The list goes on, but this can give you an idea of things to watch for.

Edited to add the following:
Please note that this information applies to the sensorless brushless systems typically found here at RC-Monster.com (BK Warrior/Micro controllers, MGM controllers, BK motors, Lehner motors, Feigao motors). These rules do not apply to brushed systems or other brushless systems, at least not in the same ways.

One other thing you should know, is that a hand/other body part.. is not an accurate measuring device to check the temperature. Knowing that something allready feels like burning when it's 120 degree F (50 C)

I wonder if we should add a new "rule" to the above list:

hot motor, hot ESC, and hot batteries = overgeared

These golden rules were defined back when cells would drop voltage a lot more under load. When an overgeared system drew too much current, the battery voltage would drop substantially, which means the voltage getting to the motor was much less. And since power is defined by V X A, heat was much less. Since ESC heat is a function of mostly current, it doesn't matter what the voltage is (to a point). The result is the battery and ESC would get hot from lots of current, but the motor would stay relatively cool because it's not seeing both high current AND voltage (just high current).

Today's batteries hold their voltage much better under heavy current draw. An overgeared system will still cause the ESC to heat up about the same (because ESC heat is a function of mostly current), but since the battery voltage does not drop as much, the motor is seeing much more voltage (and therefore power). As a result, the motor is getting hot too.

Don't know why this wasn't stickied, but it is now...