Unfortunately as vapers, we can't test these things. My multimeter can tell me almost everything I need to know regarding my gear, and thus safety, but it has no diacetyl probe insert, so for now I'm in the dark.
You do not need to stay in the dark, you know. You CAN learn to detect diacetyl/subs yourself in a couple of days, if you want to. Homebrewers (beer and wine) learn how, and so can you. It's actually fun!
For any DIY'er who is interested and has some TFA flavors already (or is willing to buy them for some experiments), give it a try:
Start with TFA Vanilla Swirl and TFA Vanilla Custard. Vanilla Swirl is the same formula as Vanilla Custard, but with the custard note ingredients removed. First lick 1-2 drops of Vanilla Swirl flavoring (not diluted) off your finger and squish it around the roof of your mouth with your tongue while you breathe in and out through your nose. Take your time. Notice how it tastes and smells. Then do the same thing with Vanilla Custard. Do them in that order; no need to wait or clean your mouth out before you do the second one. You will notice the difference, for sure. Brush your teeth, have a meal, come back later.... Open both bottles without checking the labels and sniff. You should be able to identify by sniffing which one has the diacetyl subs. If not, repeat the taste test and try again. You'll get it. When you do, move on to harder tests:
Click here to open the spec sheet on TFA's website. Grab a random TFA flavor and smell it. (Taste if you want.) Compare it to Vanilla Custard and Vanilla Swirl. Do you detect that distinctive diacetyl scent in your random flavor, the same note as in Vanilla Custard? On the website's spec sheet, click 'list' to the right of that flavor name. Look for acetoin and acetylpropionyl. See how many you can get right. Practice makes it easier. Make a note of which ones you got right and wrong. You may miss some with smaller amounts of the chemicals. Repeat the test in the previous paragraph, then re-try the ones you missed. Most people can get nearly all of them right after a round or two. Do not be surprised if you occasionally detect it in a flavor that the spec sheet says has none. Check the spec sheet again in a month or two; you're probably right. This has happened to me a few times.
You can do all of those tests in vape version instead of tasting, but it takes more time and of course you'll be inhaling diacetyl/subs.
If you've gotten to here, you probably want to avoid diacetyl/subs. If so, pick a few flavorings you have that you can detect diacetyl in. Buy the nearest flavoring that FlavourArt makes or that TFA makes without custard notes. When it comes, compare it to your old version. By now you should be able to tell them apart with a sniff, or a taste, or a vape. So now you're a diacetyl-detector.
Check other flavorings, especially Capella (anything not 'version 2) and Flavor West (especially caramel, butterscotch, toffee, creamy, and bakery flavors) and Nature's Flavors Organics. You will detect diacetyl in many. Give 'em the sniff taste. Taste a drop. If you're unsure, compare it to a similar flavor you've correctly identified before.
Go to the DIY area here and start a new thread, something like "my diacetyl-detection experiments" - and tell us how it went! I'd love to hear all about it, and we can all swap notes.
If you want to be a diacetyl-super-detector, you can go further. You can compute your personal threshold of diacetyl detection by purposely adding a known diacetyl amount to a custard-note-free flavoring, but that's a post for some other day, if anyone (as geeky as I am) is interested.
