Look out for knowing glances, avoiding eye contact or mumbling when they answer.
From the money.uk.msn.com
By the time I was middle-aged, my mother used to grumble at me to stop mumbling.
From the economist.com
He was pacing back and forth in the ring mumbling obscenities under his breath.
From the morningstaronline.co.uk
They contact their mothership by rolling their eyes and mumbling behind your back.
From the ocregister.com
Solo singers reiterate single words quickly, as if mumbling rosaries in penitence.
From the ocregister.com
People like to tell you that Marlon Brando mumbling in a dark room is good acting.
From the metro.co.uk
Ten seconds later, he knocked on the door, mumbling that he'd forgotten something.
From the latimes.com
She heard coughing and mumbling but didn't recognize the voice as Murray, she said.
From the thenewstribune.com
Depp is outstanding, mumbling his way through the creation of a new identity.
From the delawareonline.com
More examples
A soft indistinct utterance
Talk indistinctly; usually in a low voice
Grind with the gums; chew without teeth and with great difficulty; "the old man had no teeth left and mumbled his food"
Happy Feet is a 2006 computer-animated family film with music, directed and co-written by George Miller. It was produced at Sydney-based visual effects and animation studio Animal Logic for Warner Bros. ...
Mumble is a voice over IP (VoIP) application primarily designed for use by gamers, similar to programs such as TeamSpeak and Ventrilo.
Mumbles or The Mumbles - Y Mwmbwls is an area and community in Swansea, Wales which takes its name from the adjacent headland stretching into Swansea Bay. ...
(Mumbles (community)) Mumbles is a local government community made up of the electoral wards of Mayals, Newton, Oystermouth and West Cross in Swansea, Wales. Mumbles has its own elected community council consisting of eighteen councillors. At the 2001 census the population is 16,774.
The Mumbly Cartoon Show was a Saturday morning cartoon produced by Hanna-Barbera Productions which aired on ABC from 1976-77.
(The mumbles) The Mumbles were a small band that were based in Dublin, Ireland in the early 1980s. Fronted by charismatic singer, Killian Devitt, The Mumbles achieved moderate success, and had a number of successful releases in the UK and Ireland.