A good thing to take advantage of here is the number line. The number line almost universally increases from left to right. You could use that analogy to unambiguously show your user which way to increase or decrease the amount of the “thing” your application deals with.
The problem with a rotating knob (from now on the tap model) is that you could bend the number line with its center up or down leading to the ambiguity that your question is trying to solve.
Another problem with the tap model is that it is very hard to use with a mouse.
An option that is both intuitive and easy to use with a mouse is the linear increase-decrease button (from now on the sound mixer model). Think of the windows sound volume controller.
Having a horizontal line knob gives you the chance to take direct advantage of the number line.
USe a vertical line knob, and you can take advantage of the fact that “up” can be associated with “more” and “down” with “less”. Nonetheless, you can always have a side bar showing (+) or (-); 0,1,2,…, 10; -5,-4,…,0,…,4,5; to objectively show your user which way to increase/decrease whatever you want to control (a rectangular triangle becoming thicker in the plus direction also works).
For a skeumorphic knob really it depends on what kind of “thing” is going to be controlled.
Most “material” fluids work using the tap model.
For electrical or similar things however both options have been used in the real world, therefore you can have a linear skeumorphic knob in this case.
Still an option to set things can always be put into the user's hands. The solution I’m giving should in principle be used as a default only.
PS: You may want to search for the number line in the countries that use right-to-left writing (like Arabic) or up-to-down writing (like Chinese) to see if there is a different way to interpret it. However, these things can be fixed up using the operating systems globalization classes and automatically set the default for the increasing/decreasing direction.
(*): In Visual Studio it is called a Track bar; in many other things it is a slider (from comment by wizzwizz4).
EDIT: For mechanical or in this case, thermal machinery, you can use a lever as your skeumorphic knob and take advantage of the second option.