I'm sure that this is something that we have both used a ton of in our electronics related lives. Even good quality heat shrink can be applied badly. Plus, we can only guess which brand someone else has used. I just bought 4' lengths of 3/64", 1/16", 3/32", and 1/8" in my last MPJA order. The paper label on each length tells me it is "PermaSleeve" polyolefin and is UL & CSA certified @ 600V @ 125 C. But there isn't a single marking anywhere along it's length to clue you in. I often use it in conjunction with clear teflon tubing which I believe has superior dielectric properties.
As an aside, I was recently asked to order some more of the yellow silicone jacketed cable that was used for signal analysis long before I showed up. Imagine my thoughts when I learned that the 1000V rated silicone jacket surrounded a 300V rated pair. OOPS!!!