I see several approaches here, both source and binary, looking for the Java 8 API and looking for Java 8 features.
Look for Java 8 APIs
Look for regular Java 8 APIs in source code (simple)
You can parse the code and look for the APIs first introduced in Java 8 - as you suggested in your answer. The most common features of the Java 8 API are threads and a class Optional, as well as a package java.util.function. I would expect almost any project that switched to Java 8 to use these features.
Look for all Java 8 APIs in source code (advanced)
100%, API Java 8 , , API, Java 8. , , Sigtest, API ( Java).
, , . .class , . , , . ClassDep ( Apache River, ).
, . Java 7 51.0, Java 8 - 52.0, Java 9 - 53.0. javap, JDK, -v.
Java 8
, , Java 8. , Java 8, lambdas . (, , ) Java 8 .
. " Java 8 ?" 100% . , , , Java 8 API, API Java 8, Java 7.
" 8 ?" - , lambdas, 52.0, .
" API Java 8?", , . , API- - , , , - , .