You find the developer's contact information and ask them. Maybe they'll have time and be kind enough to answer.
However, as it's been mentioned in the comments, having the infromation is most likely irrelevant.
If a wood sculptor is very proficient with a chainsaw, but you're an ace with an angle grinder, you'll probably end up creating better looking wood statues using angle grinder than if you clumsily used a chainsaw. Similarly, it's not because you know they have used engine X that you'll know how to use engine X to achieve similar results.
An engine is just a tool to an end. Use the tool you're the most comfortable with to get to that end.
That being said, even if you know they used X to get result Y in game Z, if you end up with result Y too, chances are that you'll be accused of copying Z in the playstore comments for your game.