They won't be able to tell? Wtf kind of question is that. It's a possession thing now, not a source thing. If you're over 21 and live in CO, you can possess an ounce. If you're a non-resident over 21, you can possess a 1/4 ounce.
If you're under 21 and high
If you're smoking in public
If you're smoking while driving
If you're smoking then driving
If you exceed your possession limit
If you get caught purchasing weed illegally
Then you need to worry.