I suggest you look in the manifest for each library for lines like
Bundle-RequiredExecutionEnvironment: J2SE-1.3
X-Compile-Target-JDK: 1.5
Build-Jdk: 1.5.0_22
Created-By: 1.6.0_21-b07 (Sun Microsystems Inc.)
I would only take the create by if no other option appears.
The only way to know for sure is to example a class in the library (assuming all classes where built for the same version)
javap -v -classpath net\sf\trove4j\trove4j\3.0.2\trove4j-3.0.2.jar gnu/trove/list/TLongList | grep version
minor version: 0
major version: 49
A table of versions are
major minor Java platform version
45 3 1.0
45 3 1.1
46 0 1.2
47 0 1.3
48 0 1.4
49 0 1.5
50 0 1.6
51 0 1.7
52 0 1.8
For more details http://en.wikipedia.org/wiki/Java_class_file