"The USFS is probably not going to stop or bother you about the sticker since they are Federal and the permit program is it's not their program. The permits are sold and managed by the State of Colorado through Colorado Parks and Wildlife."
The UDFS rangers write tickets to ATVs, motorcycles, and side by sides that do not have OHV permits all the time. They will start enforcing it on full size rigs were applicable.
In addition CPW has staff now that their sole purpose is to ride trails and enforce the permits. I ran into them on Moody Hill last year, 2 guys on dirt bikes, and they noticed I had the permit.