You know how we all fold our arms across our chests? One generally reads that as a posture of waiting or of being cold.
When you do that, one hand ends up tucked in out of sight and the other rests on top of the opposite bicep.
More often than not, the top hand is the hand the person writes with.
Sometimes it doesn't work and there's no explaining it. Sometimes it doesn't work with older people who are likely to have attended schools where lefties were re-trained to used their right hand instead (in my experience, many of these older switched people have no memory of it). Otherwise it works most of the time.