How do I know my Samsung S8+ Model SM-G955FD is Exynos and Snapdragon?
The model number unambiguously determines the device's core configuration - if it says G955F(D) everywhere on the device (especially in Download Mode, which AFAIK can't be faked), then you can be sure that it's an Exynos variant.

Boot into Download Mode with the key combo VOL- + BIXBY + POWER, and you'll be greeted with a screen like below, where the model number will be shown after "PRODUCT NAME".



EDIT: For a more detailed breakdown of all S8+ models (full specs), check out this list from PhoneDB: Samsung Dream 2 ("Dream 2" is the development codename of S8+)