In short, you don't.
The SoftwareSerial implementation for AVR doesn't have an outbound buffer at all.
It just turns the interrupts off during each character outbound and it sends them all immediately.
This means a call to SoftwareSerial's write simply will block until all of the data you've tried to send has in fact been sent. If you can't afford to wait for your data to go out on the TX line now, then it's up to you to send it later when you can afford to wait.