How to Identify which Is the

Sounds like you blew a power tube in the left channel of your amp. Look at your power tubes - are the left channel tubes lighted when the amp is on? If not that is your problem and you must replace the left channel tube(s). You might also have to replace the fuse, and if a new tube won't bias, you might have to replace a resistor which quite frequently (in some amps) will go with the power tube.

Well it doesn't sound like your power amp - if that were it you wouldn't get anything out of your left channel. Should be the same for the preamp which would leave the problem in the CDP. You might try plugging the CDP into a different set of inputs on your preamp and see if you have the same problem with the CDP - if not then the problem is probably with the inputs on the pre-amp. If you have another source, such as a tuner or another DAC or CDP you could try it an see what happens - using the same inputs and another set of inputs. I'd guess that there might be an intermittent ground fault somewhere and that is what causes the P500 to flash on & it could be as simple as a shorting in a solder joint in an female RCA on the pre-amp or CDP (has happened to me ofter and is why I recommended using a seperate set of inputs) or a cracked solder joint on the interconnect itself. Try sub'ing in a new interconnect. Keep us posted.

Your original description of how the channel went out is very typical of what occurs when you have a sudden power tube failure (as I mentioned above). If you haven't done so, check out your amp.
You could have a short in your cable or the posts on your amps and speakers - check your cables for shorts and stray pieces of wire at the ends which could cause a short.
I have zero knowledge of the type of cables you are using BUT some cables do not go well with some amps - They can actually cause damage, its not just a matter of tone. Be sure you have an appropriate match. You might also try using another set of cables and see if that makes a difference.
Check the amp!