Hi, depending on your ISP provided modem/router you should be able to see the maximum bandwidth of your DSL by logging into the UI of the router (usually 192.168.1.1 or something similar, it's even written on the backside of the router) and navigate to (also depending on the manufacturer) to your xDSL statistics or internet status or something similar. Once you're there you'll see a lot of info such as line attenuation, noise margin, output power, etc.. All of them will be sorted in two groups called upstream and downstream (or upload and download). In those two categories you should see actual line rate or maximum bandwidth or something similar and the value is presented in Mbps or kbps.
The maximum bandwidth should always be higher that the current bandwidth that you're using (you can check that on speedtest.net)
For reference: If you're using ADSL2+ your max bandwidth will go up to 24-25Mbps, for VDSL2+ this value will go up to 65-60Mb/s,
Note that your maximum bandwidth will primarily depend on the length of your line.
I hope this helps