You mean besides that they taste differently?
Best way to tell the difference is by testing them. A custard is a sauce or a cream that includes vanilla, sugar, butter and egg yolk. Vanilla custard is a cream and stronger than Bavarian Cream. Vanilla Swirl is certainly the sweetest of the TFA vanillas, imho, and I use it as both a vanilla and a sweetener, and most often when I'm running out of head room at 30% flavoring. What I mean by this is that if I don't have enough room at 30% maximum to add both vanilla and sweetener/cotton candy, I will always use Vanilla Swirl.
TFA has some information about custard notes, diacetyl, and other ingredients that is important to read and understand:
"Introduction: I have received some calls from customers who wish to learn a bit more about some of the ingredients used in flavors. Specifically, there are 2 ingredients that have people curious. Acetyl propionyl and acetoin. Both of these ingredients are replacements for diacetyl, which is no longer being used by many (but not all) flavor manufacturers. There are some who still include diacetyl in their flavors, but it is not used as an ingredient in any of the flavors we offer. Athough acetyl propionyl and acetoin are regarded as safe for use in Food Flavors, there are studies being done as to whether there are problems with inhalation. I am no expert by any means on sensitivity and safety issues, I simply provide flavors that are Food Safe, but when customers call me with questions, I try to help in any way i can. Any time you have a flavor that has a vanilla-custardy type note to it, it will have one or both of acetyl propionyl and acetoin. There is also the possibility that there will be Diacetyl as an added ingredient, (the flavor manufacturing company that we purchase our flavors from is restricted by their insurance from using Diacetyl as an ingredient, because of employee environmental health issues, but this is not true of all flavor manufacturing companies). The flavor notes of diacetyl, acetyl propionyl and acetoin are rather obvious, and once a person learns how to recognize them, they are easy to spot. Just like once you know what cinnamon tastes like, you can usually tell if a cocoa recipe has cinnamon in it just by taste! Our vanilla custard is a good example. Pretty much by definition, a vanilla custard flavor, no matter who makes it, will have both acetoin and acetyl propionyl (or diacetyl) in it. Just like it would be hard to bake a cinnamon cookie with no cinnamon, it would be really hard (pretty much impossible) to create a vanilla custard flavor with no acetoin or acetyl propionyl (or diacetyl). If you would like to learn a bit how to tell the differences, you can compare our Vanilla Custard (which has both acetoin and acetyl propionyl), with our Vanilla Bean Ice Cream. The Vanilla Bean Ice cream does not have any acetoin in it, but otherwise is the same flavor as the Vanilla Custard. So by tasting the two, you can learn to taste the difference between them and learn what acetoin 'brings to the party'. I like to experiment by tasting flavors in Cool Whip. This is a good way to learn about ingredients. Remember that both Acetoin and Acetyl Propionyl are perfectly safe for food-use, so there is absolutely no problem at all with tasting these flavors in Cool Whip or water or any other food. Then you can taste our original French Vanilla Flavor, which has neither acetoin nor acetyl propionyl, and compare with our French Vanilla Deluxe, or the Vanilla Beach Ice Cream, or the Vanilla Custard, which has them. You can see how the flavors are different. By learning how to recognize these flavor components, you will be more able to recognize them in flavors by other manufacturers too! Sometimes only a little bit of these "custardy" notes are added, just as a subtle nuance. For example our waffle has a little bit, but not nearly as much as the Vanilla Custard. So to help people, we are marking the main flavors that we know contain these two ingredients, so that if you choose you can avoid them altogether! Unfortunately, we might not know of all the flavors that may contain tiny "trace" amounts, but we marked them as best we know."
Diacetyl is the buttery taste we recognize on microwave popcorn. To my knowledge, TFA does not use diacetyl in any of their flavorings. Two cents...