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

This Stereophile AES J-test includes a very high level signal mixed with a LSB (smallest signal) and is a great test for interface jitter and any modulation distortion.

Ex 1. Benchmark DAC3 HGC, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 24-bit TosLink data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.

https://www.stereophile.com/images/1117BDAC3fig11.jpg

There is no jitter (spurious signal) visible above -150 dbfs noise floor on the analog output. This means there is excellent interface jitter rejection.

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Ex 2. Schiit Yggdrasil, high-resolution jitter spectrum of analog output signal, 11.025kHz at –6dBFS, sampled at 44.1kHz with LSB toggled at 229Hz: 24-bit USB data (left channel blue, right red). Center frequency of trace, 11.025kHz; frequency range, ±3.5kHz.

https://www.stereophile.com/images/217Schiitfig12.jpg

There is jitter (lots of low level spurious signal at very specific tones).... probably inaudible but it is there.

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.