If I bought health insurance today, how soon could in go into effect?

Also, how do insurance companies determine whether a condition is pre-existing or not?
--------------------
Each policy is different, so you will have to ask the agent. In the case of pre-existing conditions, they usually ask you to disclose things up front. If they get a claim for a diagnosis that is often pre-existing, they will request medical records to determine whether you were previously treated for the condition.
Source