The way to know for certain is get a digital volt meter, set to to Ohms, if your clip cord isn't marked touch one lead to one of the clips, then touch the other lead to the other clip, if the number is positive...take note of what color lead was touching which end. That's your red positive clip lead mark it. Do the same thing with all your machines. If you get a negative number...then the opposite lead/post is positive. On a linear machine I don't think strike rotation makes any difference, but on swashes and old style non-linear machines it makes a large difference with hand motion vs rotation. Most machines I've tested red lead was at the bottom machine post...so when in doubt use that as a rule of thumb, as a side note on all my swashes, the opposite is true; the top machine clip post is positive.

The thing that matters most in motor life is consistency, always run them in the same polarity...red to red, or if you like it reversed keep it reversed, unintentional flip flopping it with unmarked polarity, is what makes the motor go south sooner.