아닌가? is probably the closest phrase to saying "or was it?" after a sentence to express a doubt to oneself.
If you're asking a real question, there is not special phrase. You just say 아니면(or), and ask the question the same way.
- It wasn't anything important ... or was it? = 별일 아니었어 ... 아닌가? (doubting to oneself)
- I bet your test wasn't too difficult. Or was it? = 시험 별로 어렵지 않았지? 아니면 어려웠어? (earnest question)