Is the "red dot" the same as R or L?

Red is always Positive or Right in audio as the right RCA has a red plug or a red stripe on it.

'Left' and 'Right' only really matters for the source (output) and the speakers(input). Any in-between components just need an indicator on the jack so you can match the input signal to the output signal. For example, on my preamp, I switch the L/R. The right channel signal from my CDP goes to the Left input, but it also leaves the Left output and goes to the right input of my amp. And the right output of my amp goes to my right speaker. It is pretty obvious if left and right are switched as the stage is turned inside out. It not simply flipped.

Most of us tend to think red=right, though there's no guarantee that every manufacturer implements it that way. About the only time this would really matter is if you were comparing LP and CD of the same recording, in which case you would notice that the stereo image had flipped.

Also, occasionally you get a record where the liner notes say, "Guitarist A is on the left, Guitarist B is on the right." You might go through life thinking Beck is Page and vice versa. If you really feel the need to know for sure, you can: 1) RTFM; 2) ask the manufacturer; or 3) get a test record that has some clearly labeled single-channel tracks.

Red normally means "right" or "positive"