What is the call if two colours are hit at seemingly the same time?

Im from the US, and we dont have very many places where you can play Snooker in a casual setting, so this question mostly comes from my own curiosity. What is the call for when the On-Ball and another Off-Ball are hit at what otherwise would seem like the same time? For Tournaments, It seems like the simple answer would be "Whatever the ref decides to call". But what about for a more casual setting, like those in pubs or at home? Is there a "gentleman's agreement" that it is a foul or not?