I wondered also how they would be enforced. My License and Passport both say Female. Its not like people carry around their Birth Certificate, so how would they know what sonmeone's BC says? Some states allow a BC to be changed with surgery (IL requires the surgeon to be in the US), some without (CA, DC, IA, NY, OR, UT**, VT, WA, WI**, AZ***), and some not at all (ID, KS*, OH, TN*). Without knowing where someone was born, even a genital check (which would be highly unconstitutional) wouldn't be of much value since some places will change it without surgery and New York City apparently will change it with just a letter from a therapist indicating your gender identity. Some states will issue an entirely new birth certificate and others will only amend it (sadly, I think my state is one that amends rather than replacing).
*Tennessee has the only law explicitly prohibiting gender changes on birth certificates for trans* persons. Kansas law permits "minor" changes, but the courts have ruled that changing the sex/gender for a trans* person is not "minor" and cannot be changed.
**Utah and Wisconsin state that a court order from ANY US or Canadian court is required, which would seem to leave the door open to changes without surgery depending on where the person lives.
***Arizona will accept proof of chromosomes in lieu of surgery to change a birth certificate, which would allow maybe 0.1% to 1% of the population to change without surgery.