RS-232 to RS-485 transceiver
RS-232 is single-ended, meaning that the signal is referenced to the ground, while RS-485 is differential - signal is referenced between A and B. That's before we even talk about TE/RE(DE) signalling of half-duplex.

What you need is a signal converter between single-ended (UART/RS232) to differential to even start communicating. Whether you implement it yourself or use readymade ICs, is up to you. 

If you already have converter, by default you sit with RE only and listen to the bus. Implement some sort of addressing, so that your device responds only when addressed. Only once addressed, you actually enable TE and transmit. Once done with transmit, get back to listen-only mode. Addressing/strict protocol is the only way to prevent bus clashes in buses without dedicated chip-selects.

Edit:

If you want your RE/DE triggered by the data bus, you'll need some sort of timed flip-flop, that gets set by your start bit and reset using a simple timing circuit, that acts as a watchdog for TX line. This will control your RE/DE. Then you need to delay your signal until your transceiver actually enables transmission.