If it's noted anywhere in their medical records that they smoke they'll be in trouble for a claims review. If an underwriter gets suspicious they can order a paramed and that'll catch it. If there's ever an open requirement for the client to call underwriting sometimes they'll go over general application questions - they could catch it then.
If they find out after the app is issued they could either rescind coverage or charge the correct premium and have the client remit the extra premium dating back to the effective date.
If you're talking about an occasional smoker and it's nowhere in their records they could in theory lie and get away with it. If I, as the agent, have knowledge of any tobacco usage over the past 12 months then it's going on the app. Period.
Where they're gonna get busted is filing a new claim. The doctor is always gonna go over whether or not they smoke and like telling any lie you simply have to remember all the lies. The application is long since forgotten, now it's a year later and they're filling out paperwork in conjunction with a major health claim. The doctor asks "So how long have you smoked" and the clients says "15 years." Then it's over.
When I have prospects trying to lie or figure out if they can "omit and omit" that I just never call them back. If they're gonna lie to me then they're gonna roll me under the bus when the policy gets yanked.