If you aren’t sure if those around you are vaccinated and you’re uncomfortable or unable to ask, it’s best to be cautious and wear a mask.
Source: https://www.nytimes.com/article/mask-mandates-guidelines.html (UCSF Medicine Department Chair via NY Times)
https://hawaiicovid19.com/mask-guidance/ (DOH)
https://www.cdc.gov/coronavirus/2019-ncov/travelers/face-masks-public-transportation.html (CDC)