all of the following are true:
1. some trans people can and do pass very well
2. some trans people cannot or do not want to pass at all
3. pre-transition trans people who look like their AGAB exist
4. intersex people who do and don't identify as trans both exist
5. pronouns aren't an indication of gender
6. presentation isn't an indication of gender
7. gender nonconforming cis and trans people exist
8. sometimes someone's gender literally just isn't any of your business
all of these things make it impossible to definitively, 100%, with absolute certainty, tell someone's gender without them being okay with telling you.