You can't really know. There are some tips here in the duo Facebook group:
Tell me if you can't open it I'll paste here
There is no difference in pronunciation and not a lot of tricks to know. You mainly have to memorize the spelling of the words.
Tet is used in words that originate from English (or some other language) for example "Internet-אינטרנט" "student-סטודנט" "telephone-טלפון" and so on to every Hebrew word you recognize from another language. (accept for words that are written with TH in English they will have Taf - like: Thomas=תומס
When writing verbs the root might have Tet or Tav but the things you add to the root are always Taf הלכתי, התלבשנו...
So you can remember that these verb additions will be Taf.
This seems like the most frequently asked question around here. Adjectives of definite nouns also take the definitive article. התפוז הטעים = the tasty orange.
Since טעים doesn't have the definite article here, than it's not an adjective. It's the predicate of a "to be" sentence. התפוז טעים = the orange is tasty