Yes, there is a DNA test. It requires a blood sample using a $20 kit you send for from this place. Research Assciates Laboratory, 411 McDermott Dr STE B, Allen, TX 75002-2854 972-2221
www.vetdna.com The test is around $100.
They will test a blood sample for DNA for Marek's or leucosis virus. As far as I know, this is the only way to determine if a chicken has one of these specific avian viruses.
I didn't follow through because I wasn't able to find a way to get the sample to them in under 48 hours, beyond which the sample would degrade and be unusable. USPS takes a week and I don't have access to UPS or FedEx.
I did get a PCR test done on a throat swab for a respiratory disease, which came back negative. These folks will send you the testing materials and do the test. Zoologix Inc 9811 Owensmouth Ave, STE 4, Chatsworth, CA, 91311-9547 818-717-8880
www.zoologix.com This test is also around $100.