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).
If you're interested, a good resource for Chinese characters and words is https://www.mdbg.net/chinese/dictionary.
The entry for 國籍 is at https://www.mdbg.net/chinese/dictionary?page=worddict&wdrst=1&wdqb=%E5%9C%8B%E7%B1%8D