Here are examples of some of the different graphic design styles Turnabout used on their quad releases.
In the first style they prominently advertised "QS Quadraphonic Stereo Compatible" on the front covers with the QS logo. Later on in the second style they still labelled the records as quad on the front, but a little less prominently. In the third style they stopped advertising quad on the front covers all together, but generally still mentioned it in a blurb on the back, often in the lower right by the copyright information.
In all cases the key giveaway is the use of the "QTV-S" catalog prefix number, "Quadraphonic Turnabout/Vox - Stereo", as the "TV-S" is their regular stereo. The covers were often 'bland' in design and color selection, with the use of classical paintings as the artwork. The "T" in a rhombus/diamond on the front cover is another reliable giveaway as this was the logo they were using during the quad era. If you see any other Turnabout logos, it's almost certainly not quad. When I see that diamond logo I slow down and check the catalog number for a "Q", and will also look for the rear blurb and even at the LP label if things aren't clear.
Mark Anderson's discography is of course the greatest resource, and Ebay is great even if for only window shopping: you get to see what quad album covers look like and start to commit things to memory (even if it's against your will!). Seeing so many things on Ebay has helped raise flags for me when record picking and caused me to pause or back-up when sifting through a box of records. These three photos were just lifted from Ebay by searching for "Turnabout quad", which was a very targeted and limited search and missed a bunch of offerings because those auction headings didn't include "Turnabout". There are a lot of single inventory classical quads out there in thrift stores.
And, just like with your Angel SQ boxed set, Vox put out QS encoded boxed sets. Your boxed set may not mention the "double circle" Angel logo as I don't think all of the boxed sets were transparent about it, but a lot (most?) of the individual Angel LPs had a blurb on the back divulging the secret. See below.
Welcome to the quad LP addiction! Involve Audio served as the enabler to get you hooked, and were just here as your suppliers (of info) now!