Kristin Cavallari Gives Marriage Advice to Engaged Couple, 'Don't Do It'

"Ask a silly question" ... is what Kristin Cavallari and Jay Cutler are clearly thinking in this video, which proves the writing was on the wall a couple months before they announced their divorce.

TMZ obtained this clip of the now-estranged couple at an event back in February, where they were doing a meet and greet with fans in Chicago at Kristin's Uncommon James store.

At one point, a woman who said she was newly engaged asked for advice about being newlyweds and marriage. Kristin quickly blurted out a very telling answer ... "Don't do it."

Everyone had a laugh about it -- including Jay who seems very uncomfortable sitting right next to Kristin -- but it sure seems like she was being honest, based on what we now know.

Not too long after this she and Jay officially called it quits, with him filing for divorce -- but Kristin's since said in her divorce papers ... they were close to pulling the plug late last year. They made one last-ditch effort that went south in March. The split has been anything but civil.

As we reported ... Kristin accused Jay of "marital misconduct," which we found out pertained to them not agreeing on child custody. They're also at odds about a living situation right now, as Kristin wants money freed up to buy a new place of her own.

Her fans can't say she's not honest.