On the trip back to Bangkok yesterday we went through 2 security check by police. The first on the Chiang Rai to Chiang Mai bus the second on the Chiang Mai to Bangkok bus.
Police searching for illegal Laos/Burmese and also checking for drugs in jackets etc. The checks did not seem to thorough, I and the other farang were not asked to show an ID or passport of any kind.
The police concentrated on the darker skinned people (Laotians and Burmese are generally darker skinned than Thais) and also listened for accents and language skills. No one was taken off either bus as all was in order. I guess these security checks happen every trip. Along the border with Laos workers can sneak across and work (like the man I met in Chiang Kong Som Pet who worked illegally as a driver) but Thai authorities have many ID checks more inland to help control drugs and or illegal workers from entering deeper into the kingdom of Thailand.