I am a german native speaker and a linguist but I still think there is no rule to determine the pronounciation of the german v. Some words beginning with v are pronounced like f (der Vogel) and some are pronounced like w (English v) (das Verb) At the end of a word or a syllable v is always pronounced like f because of the german Auslautverhärtung. That means we Germans can not pronounced voiced sounds (consonants) at the end of words/syllables: v -> f; d->t; b->p; g->k
In general, the letter V is pronounced /f/ in native German words and like /v/ in loanwords.
Unfortunately, you can't necessarily tell just from looking at a word whether it's a loanword or not.
But if you can recognise the word from, say, French or Italian, then it's likely to be a loanword from Latin or some Romance language.
Some Duo trees have an exercise that distinguishes personal and private. So far I have not seen It is not private, it is personal! on the German for English speakers tree:
- Ce n'est pas privé, c'est personnel ! [French]
- Det er ikke privat, det er personligt. [Danish]
- Det är inte privat, det är personligt. [Swedish]
- Itu tidak privat, itu pribadi! [Indonesian]
- 그것은 사적이 아니라, 개인적입니다! [Korean]