That is true about Spec number, sometimes they do not show up any place. I even had some that I had a friend run them through Intel's site and they didn't show up.
Intel's PCNs (Product Change Notifications) can be a useful resource for mapping unknown S-specs to model names and vice versa. You won't find "deep cover" things like rumored NSA processors in there, but you will find things like the AWS models. For example,
this PDF.
There is risk purchasing ES chips from any place and there is a reason they are engineering samples and not retails. Purchase ES chips at your own risk I say.
Agreed. Possibly not for the reasons some people think, though. A common school of thought is that Intel deliberately makes these processors with a short lifespan. That's unlikely as the board / system manufacturers are going to torture those parts with both intentional (system thermal qualification) and unintentional (board layout errors w/ shorts, excess capacitance, etc.) out-of-spec operations.
My main area of concern, even given a processor that is reported to be identical to the production version (for example, the X5698 Q5C1 vs. SLC32) is that microcode updates for the production part won't apply to the ES/QS part due to the ID being different. Long after the design is set in stone (silicon), tweaks are being made to the microcode and patch files are being created. The "first customer ship" of a processor model may be several microcode revisions newer than the base microcode in the chip. Customers never notice this because the board / BIOS partners include the patch file in the BIOS, even on those first systems. While the manufacturers may get microcode patch files for ES/QS parts, they're unlikely to make it into a release BIOS, and asking Intel "pretty please, can I have the latest microcode patch file for a processor labeled "Intel Confidential" and actually getting the file is even less likely.