Nasalization in IPA

I am learning IPA to learn the English pronunciation.

When "n" is inserted after a vowel and it is not followed by another vowel, how to know if /n/ is pronounced or it is only a mark to nasalize the vowel?

The pronunciation of "dune" in the dictionary is /djun/, and "sun" is /sʌn/. But I listened to those words and "dune" ends with an "n" sound (the tongue moves), but "sun" does not.

The IPA does not tell the nasalized phoneme in diphthongs. In "find" /faɪnd/, "a" is not nasalized, but "ɪ" is. In "saint" /seɪnt/, both "e" and "ɪ" are nasalized.

The /n/ in IPA is never a mark of nasalization of the vowel preceding it. It always means your tongue is touching the tooth ridge and air passes through the nose. Therefore it is a nasal consonant. Vowels before a nasal consonant are technically nasal through assimilation, but in English, unlike some other languages, there is no phonemic distinction between nasal and oral vowels. Therefore you should not worry about it.

Both "dune" and "sun" end with the same sound, the only difference is in the vowel. The UK english pronunciation of "dune" differs slightly from the US: /djuːn/ vs. /duːn/. The IPA for english words does not mark nasalized phonemes in diphthongs because, again, there is no phonemic distinction between nasal and oral vowels in English. Perhaps you perceive a difference in the final "n" sound because the vowel in "dune" is a long vowel? Because of assimilation, you think the "n" sound is different: the only difference is in the duration of time air passes through the nose, the position of the tongue should be (more or less) identical.

The degree of nasalization of the diphthongs in "find" and "saint" will vary from person to person, and personally I hardly nasalize them at all.