How does a garage door wall button differentiate between a door open button, and a light button?
You got me curious so I went to check my garage door opener. I have a Genie pro stealth. It shows 3.8vdc at the terminals where the switch(s) goes. The voltage at the terminals drops to 2.1vdc if I press the light switch and 0v if I press the open/close switch. So ... it uses a resistor plus the switch for the light and a just the switch for the open/close. 

Technically you could say it is a voltage divider.  

Sorry  but I was too lazy to check for a signal with a scope. I know the sensors use a square wave with a dc offset (In my case). I built an alarm for the sensors a while back.

Hope that helps.