Open the jar with Winzip or 7-zip.
Check there is a folder called META-INF that contains a file called MANIFEST.MF
Open MANIFEST.MF in a text editor.
It should be something like:
Ant-Version: Apache Ant 1.7.1
Created-By: 14.3-b01 (Sun Microsystems Inc.)
X-COMMENT: Main-Class will be added automatically by build
Look for the line Main-Class which should list the class file in the jar which contains the main() function that is to be called.
If any of the above isn't there, you haven't got an executable jar.
Edit: If that isn't it, I'd look at the Class-Path next, which probably should have something in it if you are using additional libraries.