"see if this is a coincidence or what?"

Casual/Friendly:

"Is it just a coincidence, or is there something more to this?"

"You think this is just a coincidence, or what?"

"Could be a coincidence... or is it something else?"

Curious/Suspicious:

"Is this really a coincidence, or is there something going on here?"

"Kind of makes you wonder if it's a coincidence, doesn't it?"

"Seems a little too perfect to be a coincidence — what do you think?"