[Coco] How to tell if running on a Coco3 versus 1 or 2?

[Don Barber]

Hi folks, writing some assembly and have come across a need to determine 
if the code is running on a coco3 versus a 1 or 2. Right now its mostly 
for display output purposes (checking HRWIDTH to determine which screen 
size is in use doesn't work on coco1/2s that don't have alternate screen 
sizes so need to default to 32-column behavior on the older machines).

My first instinct is to go through the Super basic unravelled book and 
find something different about the coco3 code like the destination 
location for a ram hook or interrupt vector and check for that, or maybe 
check the startup message for version 2.x, or something similar. But it 
occurs to me that someone else has probably solved this problem already. 
Any advice?

Don