Do you use a particular source to look this up or you just use your knowledge?
Wiktionary, although usually I recognise at least one component of a word/phrase as familiar (e.g. here I recognised 國 but not 籍).
It becomes very easy to guess/recognise once you know a little bit of Chinese (characters) and Vietnamese. Delve a little bit deeper into some linguistics and you'll recognise that from just the sentence "họ không biết quốc tịch của tôi" you will find that họ is derived from 戶 (Hán Việt: hộ) which can mean household/family and hence clan/surname/they. Không is obviously from 空 by association of empt(y/iness).