TXB0304: how to implement automatic direction sensing?

Both input and output buffers are always active in both directions, but the output drivers are very weak (with 1 kΩ series resistors). If a TXB I/O pin is connected to a (high-impedance) CMOS input, then the TXB is just strong enough to set the voltage at that input. If a TXB I/O pin is connected to a CMOS output, then that output is much stronger than the TXB output driver and overrides the voltage at the TXB pin.

See A Guide to Voltage Translation With TXB-Type Translators.