The latest version of struts (1.3.5) is the easiest to tell, because finally they started putting the version number as part of the jar file names.
With the earlier versions, it's a bit harder. The best way I know of to tell is to download a specific version of Struts and compare the last-modified date of the struts.jar file in that version with the last-modified date of the one you're using.
Another possible solution is to open the struts.jar file using a program like WinZip and view the MANIFEST.MF file. I took a look at a 1.1.? version and I see this information:
I learned something new today. I never thought to look in the MANIFEST.MF file for the version. It's good to know that there's an easier way than comparing dates on jar files.