Generally, it will come up as the result of a medical examination. Some insurers require that you have a physical check-up so they can assess your health before issuing a policy. They can easily test for nicotine via a urine or blood test, which will tell them if you smoke and give a rough indication of how much.
Some insurers do not require an examination, however. In these instances, they will be relying on your honesty.
While some people may think it a good idea to omit this expensive piece of information, it’s not a good idea. If you end up making a claim and the coroner rules it was a smoking-related fatality, your policy will be invalidated and your family will not receive a payout. Similarly, a life insurance provider might ask to speak with your GP to corroborate information on your application.
For smokers with expensive life insurance policies, there are ways to decrease the premiums. The best way is to quit, get in touch with your insurer and they will arrange an examination a suitable amount of time after. Once you are deemed to be less of a risk, your monthly payments should drop accordingly. If you start smoking again though, you will need to let the provider know, or risk invalidating your policy.