If you are in question, assume that the material is asbestos. If you need to be sure, bring in a consultant that is uniquely qualified to identify asbestos. This could be an experienced contractor or someone like a building inspector. These contacts can be found on the internet. Don't attempt to take a sample on your own as you could expose yourself and others in the area to asbestos. Get a qualified professional to take the sample as they will have the necessary personal protective equipment and tools to do the job. For instance, they may need to put on coveralls, gloves, and a respirator before chipping off a piece of the material and putting it in a sealed container. They can use a high-efficiency particulate air (HEPA) vacuum to contain dust particles and clean the area.   The professional will also dispose of their equipment and the waste from the HEPA vacuum as per your area's regulations. Lab tests can tell you with certainty whether a material contains asbestos or not. Take your sample to a NATA certified lab. If there is one in your area, you can drive it there. If you have to send it off in the mail, follow any regulatory guidelines for mailing asbestos. The lab will identify the material and report back to you.

Summary:
Consult someone experienced in identifying asbestos. Have a professional collect a sample. Send the sample to a verified lab.