If a class dependency is missing, how can I determine which class is missing?

I am getting the error “Could not find or load main class” even though the class is there. From this answer, I learned that the class can likely be found, but not loaded, due to missing dependencies.

Aside from manually decompiling the class, checking all its dependencies, and if those are on the classpath, and so on for each dependent class ad infinitum….is there any way to determine exactly which class is missing from the classpath therefore causing Java to be unable to load my main class?

$ java -cp ./picketbox-4.1.1.Final-redhat-1.jar org.picketbox.datasource.security.SecureIdentityLoginModule HelloWorld
Error: Could not find or load main class org.picketbox.datasource.security.SecureIdentityLoginModule
$ jar xvf picketbox-4.1.1.Final-redhat-1.jar | grep SecureIdentityLoginModule
 inflated: org/picketbox/datasource/security/SecureIdentityLoginModule.class
$ cd org/picketbox/datasource/security/
$ javap SecureIdentityLoginModule.class | grep extends
public class org.picketbox.datasource.security.SecureIdentityLoginModule extends org.picketbox.datasource.security.AbstractPasswordCredentialLoginModule {
$ javap SecureIdentityLoginModule.class | grep main
  public static void main(java.lang.String[]) throws java.lang.Exception;
$