It's not easy! When I find info from a reliable source, I just add that to my flavors spreadsheet. I don't have a list of links. FA no longer provides the info on their website, but you can find screen shots and references to the percents they used to list from 2010-2011, when they first started reformulating their flavorings. I linked to a few of those sources over on ecf. Just FYI, the last percentage of diacetyl FA listed for a few flavors: Croissant 0.074%, Malt 0.01%, Butter 1.5%, Condensed Milk 0.5%, Yogurt 1.6%, Pandoro 2%, Panettone 2%. (None were ever listed over 2%). Champagne was listed at 0.012%, and only recently reformulated for addition to the e-cig flavorings. So you can see how very little it takes for FA to add that inhalation warning.
TFA's info is on their website, though not all flavors have been tested.
http://shop.perfumersapprentice.com/specsheetlist.aspx For example, scroll down to Vanilla Custard, click 'list' on the far right. Add up the percentages for Acetoin and Acetylpyrazine (sometimes written acetyl pyrazine). That flavor has between 2-20%. They don't list with more precision than that. On that screen, you can click on any ingredient name and see a list of all TFA flavors that use it (then click 'list' for each one to see the percentages). The lowest percentage they show is "<.5%". I
suspect they do not test to the thousandth-of-percent as FA does.
Regarding Flavor West, it was a huge blowup earlier this year. Their flavorings said "diacetyl-free" right on the label and website, but a Canadian vendor paid to have an independent lab test FW Caramel Candy, and it was loaded with it. Flavor West then began removing the "diacetyl-free" claim from bottles and some pages on their website. It was discussed a lot over on ecf. Many of their other flavors are highly, highly suspect. I don't remember if/which others were lab tested, but FW has proven it's simply not reputable, IMO.
I don't know if Capellas ever formally stated that they use any diacetyl at all. They say they're removing it from some of their flavors (therefore, it was there until now, right?) They will not release any ingredient info or numbers, period. Far better diplomats than I have tried and failed.
For my own (and partner's and friends') purposes only, I rely on my own nose/stomach. If there's ~ 2% or more diacetyl in a flavoring, I can clearly smell it. Vaping it upsets my stomach; at high enough percentages, I almost get physically ill. I wrote repeatedly on ecf about FW Caramel Candy giving me that reaction before the flavoring was tested. I knew the same about several TFA flavors before their test results were available. That nauseousness hasn't failed me yet - always means there's lots of diacetyl/subs. If it's low enough, I can't tell, so some slip past me. Several of TFA's with custard notes do not give me a reaction at all, but I've never gotten nauseous from one and found it to have no/low custard notes. I have 20 or so Capellas I "know" contain over 2% diacetyl, just from my reaction though. I was using them in coffees for a while, but now even that makes me a little woozy. The diacetyl smell in some is so strong, I smell it even in public, in other vapers' exhaled vapor. Anyway, my nose/stomach have proven reliable for TFA and FA flavorings that have published numbers, so I rely on my senses for others, too. It's totally anecdotal, opinionated evidence, so everyone can feel free to totally ignore this paragraph. My catchphrase online is always "likely contains some custard notes" - means it makes me ill, but without lab tests, I can't guarantee I'm right.
