As of the 1.1 update of MCPE, you cannot. It updates everytime you type on the sign now, not when it's finished.
You can have something along the line of a delayedtask, basically checking if it hasent been edited for example 1 min and fire the necessary events like usual
If Mojang don't give us an event for sign completion soon it's probably best to switch to a different mechanism whereby you activate signs by command/tap after they've been written.
Ah, good idea. I was going about that in a different approach (checking the sign as it was typed but delaying the messages) but I realized I could just check it all once the player smacks the sign... Thank you for guiding me towards my epiphany