There are life insurance policies available that do not require any type of examination, including testing for illegal substances. This type of life insurance may be limited in payout, or have substantially higher premiums to maintain the policy, but they are available to everyone who passes a simple questionnaire regarding their health and living habits.
For other types of life insurance, blood tests will be part of your examination process, and will be used to test for illegal drugs as well as existing medical conditions. Testing positive for marijuana does not void your eligibility for life insurance, but it may result in higher premiums or special clauses that exclude drug related death. Not all companies will test for marijuana, and of those which do, the drug is regarded differently from one company to another. As a rule, users of marijuana are not penalized as heavily as cigarette smokers, with more and more companies taking this view as medicinal marijuana use becomes more common.
When you are asked whether or not you use illegal drugs, and you will be, the best course of action is to be honest and open. Admitting that you use pot to an insurance company is confidential information, as are the medical tests, so you do not need to worry about being turned in to law enforcement. Additionally, by admitting that you use the drug, the insurance company will not be able to deny the claim later based on the presence of such chemicals in your bloodstream, unless specific exclusions have been written into your policy.