You say something to someone (or heard something) but don't remember what language it was in because you were only focused on the meaning.
You start a sentence in one language and finish in another or you intermingle phrases and words from both within a sentence or a story (code-switching).
You have dreams in both languages.
You find it hard to answer the question "Which language do you prefer?"
These are only a few of the things that I could come up with. I invite all my readers to add to these or suggest variations on the ones I have put here.
I 'm looking forward to your comments!