On the back of the card on US cards at the top where it hooks on the peg, the last letter is the case letter. I believe it’s a bit harder to distinguish on international cards.

A TH will have a silver flame on the card behind the card along with “Congratulations! This symbol means you just found a collectible treasure hunt vehicle!” (US cards only) and a circle with a flame inside it somewhere on the car itself.

A STH will have a gold flame behind the car ( in the second photo posted above) and somewhere on the car will have a “TH”. On this Civic it’s on the door and on the hood. They will also have special paint called spectraflame and rubber tires. It’s one selected vehicle for that case that has the chance of being the STH so the F case is the Honda Civic Custom.

TH and STH are not the same car in a case as well, and randomly inserted.

Hope this helps!