How can I tell if I need a better clock for my DAC?

the analogy that could relate would be , everyone has stress in their lives but life is so much better when you reduce that stress to a minimum.

@tooblue 

I agree with you. But in using your analogy, there is more than one way to reduce stress. For instance, one could take prescription medications, practice yoga, or my favorite, listen to music of choice. Similarly, there is more than one way to reduce (or eliminate?) jitter.

I don't see the rationale behind trying (whether successful or not) to reduce jitter at the transport (i.e have better clocking at the transport), because a good DAC will re-clock anyway. It is conceivable (not saying this is the case, although it wouldn't surprise me if it is) that a quality DAC will perform its voodoo (jitter reduction, re-clocking, whatever) regardless of the signal it receives and essentially "undo" or "disregard" the voodoo that was performed prior to it receiving the signal.

Clocking before the DAC can make a huge difference. Not speaking on theory, but experience, which matches many other’s experience as well. Too many voice their opinions, often based on theory, or just a false or incomplete understanding of how things really work, without actual experience. I’m not an expert, but know what I hear. I already have a good DAC, but added an ISO Regen and SOtM clocking device and it made a significant improvement to the sound. (And linear power supplies to the ISO ansd SOtM which brought it up yet another level.) More weight, more body, resolution, improved timbre, more "analog" (though I dislike that term). Of course you also need a downstream system capable of revealing these improvements. Many or most DACs will benefit from these devices, incuding super high-end ones. I’ve heard a few situations where there was not consistent improvement among users, such as PS Audio and Exasound DACs and maybe a few others. Likely all DACs under $2-3000 will benefit and many more over this amount. I’m not really interested in those who say why not buy a better DAC to begin with, or "USB is flawed, why bother", or especially "expectation bias" (that’s BS). Hey it’s my money and I’m having fun. If things don’t work, I sell them, but that’s not the case for these devices.
Plenty more info on this topic on the Computer Audiophile forums where users share their actual experiences and what devices brought about (or didn’t, as some work better than others) these improvements.

a clock for your what??

Clocking before the DAC can make a huge difference.

Agree. I use a PS Audio PWT transport which, as many of you know, reads the CD and loads it into a buffer. It then clocks the data and sends out a timed bitstream to an external DAC.
This is why a good cable is needed to preserve the reduced jitter signal.

Interface jitter is a fact of life. It doesn’t matter what you do. The only complete solution to interface jitter is to select a DAC device that ignores the incoming clock and uses it own internal asynchronous clock. Note that this will still leave you with the intrinsic jitter of the device itself but at least you won’t have to worry about fancy cables or what length to use.

Alternatively, reclockers and a random selection of cables might help reduce interface jitter if you want to stick with a DAC that does a poor job of eliminating incoming jitter itself...kind of a dogs breakfast of a solution as you can never totally get rid of interface jitter and you would not even know if you had.