I've encountered this in multiple places:
A game script I'm translating has a psychic exclaim "はっきりせんかい!" right before having a vision of the protagonist's future.
A tweet "暖かいのか寒いのかハッキリせんかい"
Title of this blogpost
I'm assuming はっきり is "clearly" in these cases, but what is せんかい supposed to be?