Older, analog phone cable tends to be flat instead of round like an ethernet cable, and less thick, although this is not a certain way to distinguish the two.
A more certain way to distinguish phone from ethernet cable is to look at the plug on the end. A phone cable has a narrower (RJ-11) plug, and there are only four wires which connect into the end. An ethernet cable's plug (RJ-45) is wider and has eight wires connecting into the end. In addition, Ethernet cables will usually have the words CAT-5 or CAT-5e stamped onto the cord.
Modern, digital phone cables are identical to data cables.
If you have questions as to whether a cable is a valid ethernet cable, bring the cable by the Service Desk in the basement of the Science Center during our hours of operation and a staff member would be happy to check the cable for you.