Wilf Wolf's Party

"Okay fine" Yani replies, "I do have some secret hidden treasure, but I forgot where it is"
"really? You forgot? That's convenient."
" Well, kind of forgot."
"What does that mean?"
"Here, I'll show you" Yani takes Wilf to a pile of 5 rocks. "I hide 1 million dollars worth of gold in one of these rocks but, here's the embarrassing thing, I forgot which one had the gold"
"Why not just break them all?" Wilf asks
"Well, that's the other thing. I booby trapped the decoy rocks, so if you open those ones, an explosive detonates. So if I were you I'd just head back, it's too risky."
Wilf thinks for a moment, and then pulls out his pickaxe and hits....