You can see what speed your getting by logging into the modem. Been a while since I had DSL but there is something in there that is something like Uplink and downlink speed. These are set in stone so they don't always represent what you're getting, but will tell you what Verizon is actually rating your connection speed at. The Downlink speed should say 15,000kbps or around that figure if it's like other modems I've seen in the past. Then you use a speed test to see how close you're getting to that. Like I said, been a while since I messed with DSL but usually that info is on the home page of century link modems.
One way to get a business like that to shape up is to file a BBB complaint. Bigger companies try to keep their rating up and you can get responses when you wouldn't otherwise. Something to consider.