You indeed wouldn't know for sure, you'd just have to guess based on the encounter/feel and whether or not you think you've encountered a boss.

Their guess would be more accurate if they have experience with previous dungeons of similar architecture.

Frieren did mention that it felt different to a dungeon that has supposedly never been cleared. She didn't specify what felt off, but I'm guessing that it actually has been cleared and is set up for exam purposes.