Some nightclubs in the UK (Preston) to have booths that offer screening for your Class A substances for purity and/or adulteration. Is this something you would trust to use for a variety of reasons or not? Police in the area have weighed in and stated they would not target anyone who used the service.
They tested out a similar service at a couple of festivals in the UK this summer. Seemed to work well and there was no interference from police.
Personally I think its a very good idea, and more places should implement drug-testing services. They could save someone's life =)