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

In layman’s terms, you are referring to jitter correction. Jitter refers to the timing of the data stream (bits, or 0’s and 1’s) and insuring that they all arrive at the same time for processing. The Synchro-mesh and related products insure this correct timing plus can reject noise.

No CDP is jitter-free. The design and implementation will determine how well a standalone component or dac/transport will correct jitter. A manufacturer is also dealing with a price point of the component which includes the quality of electronics, vibration control of the transport, the power supply, and function of the laser.

Other members, including Steve N. (audioeng) can explain jitter correction in detail. Steve can explain measuring jitter since that’s what he does.
I thought I might start things off with the basics since it is complicated subject matter.

....No CDP is jitter-free...

And as you read enough on this forum you'll likely conclude that the interconnect cable between the transport and DAC also introduces jitter, and perhaps other items.

This is why I've always maintained that one must rely on their DAC to do its job and reduce or eliminate jitter given its re-clocking and/or other proprietary technology. And in this context, when I write "rely on their DAC to do its job and reduce or eliminate jitter", I don't believe this is a stretch as far as technology capabilities are concerned. This is why many DAC manufacturers proudly advertise this is what their DAC is doing, among other things. Frankly, I don't understand why there is so much hullabaloo regarding jitter. Apparently, it's as ubiquitous as air anyway so at some level you just have to live with it.

@gdhal , 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.

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.