How to correctly determine that an object is a lambda?

There is no official way to do this, by design. Lambdas are part of the language; and are integrated into the type system through functional interfaces. There should be no need to distinguish a Runnable that began life as a lambda, a named class, or an inner class -- they're all Runnables. If you think you have to "deal with lambda" by taking apart the class file, you're almost certainly doing something wrong!