You could do it one of a couple of easy ways.
1. Post a picture here and we can try to identify it. There are some here who are pretty catfish/pleco savvy
2. Check out one of these sites:
http://www.plecofanatics.com/ or
http://www.planetcatfish.com/Common plecos can get very large, upwards of 2' and 3' isn't impossible if they have enough room. If you have one you might want to look into getting one of the rarer, smaller varieties. You'll find plenty of ones to pick from from the sites listed above.
The plecofanatics site has a changing top banner that seems to change every time you open it. One of these rotating banners will display people holding very large plecos so you'll see what i mean when i say they get very large. If you keep hitting refresh on that first page you should get those pictures eventually.