It's almost impossible to pull this off, but if you're super worried about it, what you do is pay a fee - $75 last I checked - to the medical information bureau, and mail in your proof of identity (because you can only do this for yourself). They send you a report of all life insurance companies that have received an APPLICATION on your life, and then you call each one of them.
You can do this every month if you want.
There's no "free" online website, where you can pop in a name, and find out if life insurance exists. It's private information, protected by the privacy laws.