Don't ask guests about their exes.
They call it a break-up because it's broken, but that doesn't mean all exes part on bad terms. That said, it's probably a good idea to avoid the topic of past relationships all together, just in case the separation wasn't cordial.
For example, Bull recently helped his ex-wife battle wrongful death charges filed against her. Fans were able to get a sneak a peek at what things might've been like when they were a couple, but if the divorced pair broke bread together, it'd be completely inappropriate for a relative stranger to ask about the sordid details of their divorce.
Watch Bull on Tuesdays at 9/8c on CBS and CBS All Access.